Exhibit 99.1

ALTERRA CAPITAL REPORTS SECOND QUARTER 2011 RESULTS

Net Operating Income of $0.37 per Diluted Share

Diluted Book Value per Share Increase of 2.5%

HAMILTON, BERMUDA, August 2, 2011—Alterra Capital Holdings Limited (NASDAQ: ALTE; BSX: ALTE.BH) (“Alterra”) today reported net income of $32.6 million, or $0.30 per diluted share, for the second quarter of 2011, compared to net income of $103.4 million, or $1.13 per diluted share, for the same quarter of 2010.

Net operating income for the second quarter of 2011 was $39.6 million, or $0.37 per diluted share, compared to net operating income of $58.8 million, or $0.64 per diluted share, for the same quarter of 2010. Annualized net operating return on average shareholders’ equity for the second quarter of 2011 was 5.7%.

For the six months ended June 30, 2011, Alterra reported a net loss of $14.1 million, or $0.13 per diluted share, compared to net income of $139.8 million, or $1.88 per diluted share, for the same period of 2010. Net operating income for the six months ended June 30, 2011 was $14.8 million, or $0.14 per diluted share, compared to net operating income of $99.5 million, or $1.33 per diluted share, for the same period of 2010. Annualized net operating return on average shareholders’ equity for the six months ended June 30, 2011 was 1.1%.

Alterra’s 2010 results include the results of operations for the former Harbor Point Limited (“Harbor Point”) companies from May 12, 2010. Accordingly, a comparison of Alterra’s gross premiums written and other results of operations for current and prior periods are not meaningful. Selected pro forma combined results of operations for periods prior to the merger for the reinsurance segment are provided in Alterra’s second quarter financial supplement available on Alterra’s website at www.alterracap.com.

W. Marston (Marty) Becker, President and Chief Executive Officer of Alterra, said: “Alterra achieved a solid $39.6 million of net operating income in the second quarter despite recognizing over $49 million of property catastrophe net losses. In a year that continues to be heavily influenced by the size and frequency of major property catastrophe events, Alterra’s diversified underwriting platform has performed very well. Net operating earnings for the first six months of the year are in positive territory and our capital position has improved over the most recent quarter, putting Alterra in what we believe is a strong position going into the U.S. hurricane season.”

Second quarter 2011 results for Alterra include:

• Property and casualty gross premiums written of $563.0 million, representing an increase of $164.8 million, or 41.4%; net premiums written of $426.5 million, representing an increase of $108.0 million, or 33.9%; and net premiums earned of $348.1 million, representing an increase of $55.6 million, or 19.0%; each as compared to the same quarter of 2010. These increases primarily reflect the impact of the merger with Harbor Point - whose premiums are not included for the entire 2010 quarter;

• A combined ratio on property and casualty business of 93.7% compared to 83.3% for the same quarter of 2010;

• Property catastrophe event and significant per-risk net losses of $50.5 million ($49.6 million net of reinstatement premiums) compared to $20.3 million in the same quarter of 2010. Losses net of reinsurance and reinstatements include $42.5 million for natural disasters in the U.S., which comprise the series of tornadoes, other severe weather and flooding along the Mississippi River during the second quarter of 2011. The remainder of the net losses relate to increased loss estimates on first quarter loss events, which remain within previously announced ranges. Property catastrophe losses principally affected the reinsurance and Alterra at Lloyd’s segments;

• Net favorable development on prior years’ loss reserves of $48.5 million, or 14.0 combined ratio points, compared to $24.1 million, or 8.3 combined ratio points, in the same quarter of 2010;

• Net investment income of $59.7 million compared to $53.3 million in the same quarter of 2010,

an increase of 12.0%; and

• Net operating income of $39.6 million, or $0.37 per diluted share, representing an annualized net operating return on average shareholders’ equity of 5.7%.

Gross premiums written from property and casualty underwriting for the second quarter of 2011 were $563.0 million, generated by the segments as follows: insurance - $141.3 million, an increase of $2.2 million, or 1.6%; reinsurance - $248.8 million, an increase of $129.2 million, or 108.0%; U.S. specialty - $99.4 million, an increase of $8.7 million, or 9.6%; and Alterra at Lloyd’s - $73.5 million, an increase of $24.7 million, or 50.6%; each as compared to the same quarter of 2010.

Combined ratios for the second quarter of 2011 by segment were 70.8% for insurance, 98.0% for reinsurance, 101.1% for U.S. specialty and 93.8% for Alterra at Lloyd’s.

Results for the six months ended June 30, 2011 include:

• Property and casualty gross premiums written of $1,190.4 million, representing an increase of $421.9 million, or 54.9%; net premiums written of $916.5 million, representing an increase of $380.9 million, or 71.1%; and net premiums earned of $727.6 million, representing an increase of $241.6 million, or 49.7%; each as compared to the same period of 2010. These increases primarily reflect the impact of the merger with Harbor Point - whose premiums are not included for the entire 2010 period;

• A combined ratio on property and casualty business of 103.5% compared to 86.2% for the same period of 2010;

• Property catastrophe event and significant per-risk net losses of $165.9 million ($155.8 million net of reinstatement premiums) compared to $29.9 million in the same period of 2010. Property catastrophe losses principally affected the reinsurance and Alterra at Lloyd’s segments;

• Net favorable development on prior years’ loss reserves of $78.7 million, or 10.9 combined ratio points, compared to $41.2 million, or 8.5 combined ratio points, in the same period of 2010;

• Net investment income of $117.4 million compared to $101.7 million in the same period of 2010, an increase of 15.5%;

• A loss of principal of $25.0 million on a catastrophe bond investment triggered by the Japan earthquake and tsunami, included within net realized and unrealized losses on investments; and

• Net operating income of $14.8 million, or $0.14 per diluted share, representing an annualized net operating return on average shareholders’ equity of 1.1%.

Gross premiums written from property and casualty underwriting for the six months ended June 30, 2011 were $1,190.4 million, generated by the segments as follows: insurance - $212.5 million, an increase of $4.1 million, or 2.0%; reinsurance - $623.8 million, an increase of $349.3 million, or 127.3%; U.S. specialty - $169.8 million, an increase of $5.2 million, or 3.1%; and Alterra at Lloyd’s - $184.3 million, an increase of $63.3 million, or 52.4%; each as compared to the same period of 2010.

Combined ratios for the six months ended June 30, 2011 by segment were 76.7% for insurance, 106.0% for reinsurance, 100.1% for U.S. specialty and 126.9% for Alterra at Lloyd’s.

Balance Sheet

Total invested assets, including cash and cash equivalents, were $7,952.7 million as of June 30, 2011, an increase of $91.4 million from December 31, 2010. As of June 30, 2011, 96.6% of the fixed maturities portfolio (by carrying value) was investment-grade, an increase from 96.0% as of December 31, 2010. As of June 30, 2011, the weighted average book yield of Alterra’s cash and fixed maturities portfolio was 3.33% and the weighted average duration was 4.1 years.

Under the Board-approved share repurchase authorization, Alterra repurchased 311,000 common shares during the second quarter of 2011 at an average price of $21.51 per share for a total of $6.7 million. Share repurchases under the Board-approved share repurchase authorization for the six months ended June 30,

2011 were 6,497,150 common shares at an average price of $21.89 per share for a total of $142.2 million. As of June 30, 2011, $185.5 million remained under the Board-approved share repurchase authorization.

Shareholders’ equity was $2,793.1 million as of June 30, 2011, an increase of 2.6% from March 31, 2011. Diluted book value per share as of June 30, 2011 was $25.98, an increase of 2.5% from March 31, 2011.

A copy of Alterra’s second quarter financial supplement is available on Alterra’s website at www.alterracap.com.

Alterra will host a conference call on Wednesday, August 3, 2011 at 10:00am (EDT) to discuss these results and related matters with interested investors and shareholders. The conference call can be accessed via telephone by dialing 1-888-713-4217 (toll-free U.S.) or 1-617-213-4869 (international) and using access code 53692386. A live broadcast of the conference call will also be available through Alterra’s website at www.alterracap.com.

Alterra Capital Holdings Limited is a global enterprise dedicated to providing diversified specialty insurance and reinsurance products to corporations, public entities and property and casualty insurers.

Non-GAAP Financial Measures

In presenting Alterra’s results, management has included and discussed net operating income, net operating income per diluted share, annualized net operating return on average shareholders’ equity and diluted tangible book value per share. These measures are “non-GAAP financial measures” as defined in Regulation G. Management believes that these non-GAAP financial measures, which may be defined differently by other companies, allow for a more complete understanding of Alterra’s business. These measures, however, should not be viewed as a substitute for those measures determined in accordance with U.S. GAAP. The reconciliation of these measures to their respective most directly comparable U.S. GAAP financial measures is presented in the attached financial information in accordance with Regulation G.

Cautionary Note Regarding Forward-Looking Statements

This release includes forward-looking statements that reflect Alterra’s current views with respect to future events and financial performance. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from those suggested by such statements. For further information regarding cautionary statements and factors affecting Alterra’s future results, please refer to the most recent reports on Form 10-K and Form 10-Q and other documents filed by Alterra with the SEC. Alterra undertakes no obligation to update or revise publicly any forward-looking statement whether as a result of new information, future developments or otherwise.

ALTERRA CAPITAL HOLDINGS LIMITED

CONSOLIDATED BALANCE SHEETS

(Expressed in thousands of United States Dollars, except per share and share amounts)

	June 30, 2011		December 31, 2010
	(unaudited)
ASSETS
Cash and cash equivalents	$	987,057	$	905,606
Fixed maturities, trading at fair value		218,481		244,872
Fixed maturities, available for sale at fair value		5,403,312		5,392,643
Fixed maturities, held to maturity at amortized cost (fair value $1,049,782)		993,691		940,104
Other investments, at fair value		350,193		378,128
Accrued interest income		70,777		75,414
Premiums receivable		870,287		588,537
Losses and benefits recoverable from reinsurers		1,058,724		956,115
Deferred acquisition costs		171,517		111,901
Prepaid reinsurance premiums		215,392		149,252
Trades pending settlement		9,168		32,393
Goodwill and intangible assets		58,094		59,076
Other assets		90,671		83,247
Total assets	$	10,497,364	$	9,917,288
LIABILITIES
Property and casualty losses	$	4,230,290	$	3,906,134
Life and annuity benefits		1,317,527		1,275,580
Deposit liabilities		147,428		147,612
Funds withheld from reinsurers		122,705		121,107
Unearned property and casualty premiums		1,203,024		905,487
Reinsurance balances payable		137,910		102,942
Accounts payable and accrued expenses		104,917		99,680
Trades pending settlement		—		—
Senior notes		440,482		440,476
Total liabilities		7,704,283		6,999,018
SHAREHOLDERS’ EQUITY
Common shares (par value $1.00) 105,794,521 (2010 - 110,963,160) shares issued and outstanding		105,795		110,963
Additional paid-in capital		1,911,979		2,026,045
Accumulated other comprehensive income		132,218		98,946
Retained earnings		643,089		682,316
Total shareholders’ equity		2,793,081		2,918,270
Total liabilities and shareholders’ equity	$	10,497,364	$	9,917,288
Book value per share	$	26.40	$	26.30
Diluted book value per share	$	25.98	$	25.99
Diluted tangible book value per share [a]	$	25.44	$	25.46
Diluted shares outstanding		107,500,490		112,290,968

[a] Non-GAAP financial measure as defined by Regulation G.

ALTERRA CAPITAL HOLDINGS LIMITED

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (Unaudited)

(Expressed in thousands of United States Dollars, except per share and share amounts)

	Three Months Ended June 30,						Six Months Ended June 30,
	2011			2010			2011			2010
REVENUES
Gross premiums written	$	563,907		$	398,981		$	1,191,755		$	770,120
Reinsurance premiums ceded		(136,626	)		(79,715	)		(273,983	)		(232,935	)
Net premiums written	$	427,281		$	319,266		$	917,772		$	537,185
Earned premiums	$	443,008		$	391,723		$	932,270		$	696,720
Earned premiums ceded		(94,067	)		(98,463	)		(203,442	)		(209,220	)
Net premiums earned		348,941			293,260			728,828			487,500
Net investment income		59,665			53,277			117,431			101,667
Net realized and unrealized losses on investments		(5,774	)		(14,786	)		(24,592	)		(8,364	)
Total other-than-temporary impairment losses		(187	)		(1,167	)		(1,311	)		(1,865	)
Portion of loss recognized in other comprehensive income (loss), before taxes		(166	)		867			(71	)		1,145
Net impairment losses recognized in earnings		(353	)		(300	)		(1,382	)		(720	)
Other income		591			275			1,906			619
Total revenues		403,070			331,726			822,191			580,702
LOSSES AND EXPENSES
Net losses and loss expenses		211,133			159,817			515,539			284,782
Claims and policy benefits		15,570			13,943			30,280			31,602
Acquisition costs		64,680			48,798			135,288			73,042
Interest expense		10,630			7,916			19,089			12,858
Net foreign exchange gains (losses)		3,090			(634	)		2,212			(3,086	)
Merger and acquisition expenses		—			(54,570	)		—			(49,826	)
General and administrative expenses		69,659			50,649			140,862			87,177
Total losses and expenses		374,762			225,919			843,270			436,549
INCOME (LOSS) BEFORE TAXES		28,308			105,807			(21,079	)		144,153
Income tax (benefit) expense		(4,327	)		2,360			(7,027	)		4,325
NET INCOME (LOSS) [a]		32,635			103,447			(14,052	)		139,828
Change in net unrealized gains and losses on fixed maturities, net of tax		42,820			60,525			28,234			94,656
Foreign currency translation adjustment		165			(7,629	)		5,038			(17,369	)
COMPREHENSIVE INCOME [a]	$	75,620		$	156,343		$	19,220		$	217,115
Net income per share	$	0.31		$	1.14		$	(0.13	)	$	1.90
Net income per diluted share	$	0.30		$	1.13		$	(0.13	)	$	1.88
Net operating income per diluted share [b]	$	0.37		$	0.64		$	0.14		$	1.33
Weighted average shares outstanding - basic		105,604,786			91,042,301			106,385,007			73,779,447
Weighted average shares outstanding - diluted		107,111,909			91,661,430			106,385,007			74,524,919

[a] Includes the results of Harbor Point Limited from May 12, 2010.

[b] Non-GAAP measure as defined by Regulation G.

ALTERRA CAPITAL HOLDINGS LIMITED

CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS’ EQUITY (Unaudited)

(Expressed in thousands of United States Dollars)

	Six Months Ended June 30
	2011			2010
Common shares
Balance, beginning of period	$	110,963		$	55,867
Issuance of common shares, net		1,406			64,983
Repurchase of common shares		(6,574	)		(2,207	)
Balance, end of period		105,795			118,643
Additional paid-in capital
Balance, beginning of period		2,026,045			752,309
Issuance of common shares, net		610			1,417,316
Stock based compensation expense		22,587			30,241
Repurchase of common shares		(137,263	)		(42,166	)
Balance, end of period		1,911,979			2,157,700
Accumulated other comprehensive income
Unrealized holding gains (losses) on investments:
Balance, beginning of period		118,197			36,791
Holding gains on available for sale securities arising in period [a]		31,281			99,001
Net realized gains on available for sale securities included in net income [a]		(3,118	)		(3,200	)
Portion of other-than-temporary impairment losses recognised in other comprehensive income [a]		71			(1,145	)
Balance, end of period		146,431			131,447
Cumulative foreign currency translation adjustment:
Balance, beginning of period		(19,251	)		(11,360	)
Foreign currency translation adjustments		5,038			(17,369	)
Balance, end of period		(14,213	)		(28,729	)
Total accumulated other comprehensive income		132,218			102,718
Retained earnings
Balance, beginning of period		682,316			731,026
Net (loss) income		(14,052	)		139,828
Dividends		(25,175	)		(322,948	)
Balance, end of period		643,089			547,906
Total shareholders’ equity	$	2,793,081		$	2,926,967

[a] Net of tax

ALTERRA CAPITAL HOLDINGS LIMITED

CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited)

(Expressed in thousands of United States Dollars)

	Six Months Ended June 30
	2011			2010
OPERATING ACTIVITIES
Net (loss) income	$	(14,052	)	$	139,828
Adjustments to reconcile net (loss) income to net cash provided by operating activities:
Stock based compensation		22,587			30,241
Amortization of premium on fixed maturities		9,801			4,798
Accretion of deposit liabilities		3,176			2,754
Net realized and unrealized losses on investments		24,592			8,364
Net impairment losses recognized in earnings		1,382			720
Negative goodwill gain		—			(95,788	)
Changes in:
Accrued interest income		4,724			5,133
Premiums receivable		(278,695	)		150,598
Losses and benefits recoverable from reinsurers		(99,028	)		(4,706	)
Deferred acquisition costs		(58,894	)		(22,681	)
Prepaid reinsurance premiums		(65,487	)		(25,170	)
Other assets		(8,544	)		6,412
Property and casualty losses		304,109			(212,784	)
Life and annuity benefits		(31,203	)		(19,532	)
Funds withheld from reinsurers		1,598			(19,754	)
Unearned property and casualty premiums		293,029			94,146
Reinsurance balances payable		34,849			146,084
Accounts payable and accrued expenses		4,761			(14,882	)
Cash provided by operating activities		148,705			173,781
INVESTING ACTIVITIES
Purchases of available for sale securities		(1,153,670	)		(1,030,431	)
Sales of available for sale securities		709,314			264,405
Redemptions/maturities of available for sale securities		485,128			527,854
Purchases of trading securities		(28,065	)		(27,913	)
Sales of trading securities		24,563			1,153
Redemptions/maturities of trading securities		35,969			12,975
Purchases of held to maturity securities		(2,580	)		(14,682	)
Redemptions/maturities of held to maturity securities		18,251			16,570
Net purchases/sales of other investments		5,532			59,897
Acquisition of subsidiary, net of cash acquired		—			446,819
Cash provided by investing activities		94,442			256,647
FINANCING ACTIVITIES
Net proceeds from issuance of common shares		2,016			1,075
Repurchase of common shares		(143,837	)		(44,373	)
Dividends paid		(25,175	)		(321,396	)
Additions to deposit liabilities		583			2,997
Payment of deposit liabilities		(3,643	)		(9,873	)
Cash used in financing activities		(170,056	)		(371,570	)
Effect of exchange rate changes on foreign currency cash and cash equivalents		8,360			(19,452	)
Net increase in cash and cash equivalents		81,451			39,406
Cash and cash equivalents, beginning of period		905,606			702,278
CASH AND CASH EQUIVALENTS, END OF PERIOD	$	987,057		$	741,684

SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:

Interest paid totaled $14,382 and $9,918 for the six months ended June 30, 2011 and 2010, respectively.

Income taxes paid totaled $165 and $729 for the six months ended June 30, 2011 and 2010, respectively.

ALTERRA CAPITAL HOLDINGS LIMITED

SCHEDULE OF SUPPLEMENTAL UNDERWRITING DATA–THREE MONTHS ENDED JUNE 30, 2011 (Unaudited)

(Expressed in thousands of United States Dollars)

	Property & Casualty															Life & Annuity			Corporate			Consolidated
	Insurance			Reinsurance			U.S. Specialty			Alterra at Lloyd’s			Total			Reinsurance
Quarter Segment Information:
Gross premiums written	$	141,255		$	248,754		$	99,448		$	73,520		$	562,977		$	930		$	—		$	563,907
Reinsurance premiums ceded		(56,002	)		(28,247	)		(27,358	)		(24,918	)		(136,525	)		(101	)		—			(136,626	)
Net premiums written	$	85,253		$	220,507		$	72,090		$	48,602		$	426,452		$	829		$	—		$	427,281
Earned premiums	$	97,296		$	209,212		$	73,840		$	61,730		$	442,078		$	930		$	—		$	443,008
Earned premiums ceded		(44,338	)		(12,305	)		(22,783	)		(14,540	)		(93,966	)		(101	)		—			(94,067	)
Net premiums earned		52,958			196,907			51,057			47,190			348,112			829			—			348,941
Net losses and loss expenses		(28,992	)		(125,592	)		(32,709	)		(23,840	)		(211,133	)		—			—			(211,133	)
Claims and policy benefits		—			—			—			—			—			(15,570	)		—			(15,570	)
Acquisition costs		361			(44,366	)		(9,644	)		(10,909	)		(64,558	)		(122	)		—			(64,680	)
General and administrative expenses		(8,859	)		(22,937	)		(9,289	)		(9,536	)		(50,621	)		(259	)		—			(50,880	)
Other income		139			548			—			165			852			(23	)		—			829
Underwriting income (loss)	$	15,607		$	4,560		$	(585	)	$	3,070		$	22,652			n/a			—			n/a
Net investment income																	12,545			47,120			59,665
Net realized and unrealized losses on investments																	(1,299	)		(4,475	)		(5,774	)
Net impairment losses recognized in earnings																				(353	)		(353	)
Corporate other income																				(238	)		(238	)
Interest expense																				(10,630	)		(10,630	)
Net foreign exchange losses																				(3,090	)		(3,090	)
Corporate general and administrative expenses																				(18,779	)		(18,779	)
(Loss) income before taxes																$	(3,899	)	$	9,555		$	28,308
Loss ratio [a]		54.7	%		63.8	%		64.1	%		50.5	%		60.7	%
Acquisition cost ratio [b]		(0.7	%)		22.5	%		18.9	%		23.1	%		18.5	%
General and adminstrative expense ratio [c]		16.7	%		11.6	%		18.2	%		20.2	%		14.5	%
Combined ratio [d]		70.8	%		98.0	%		101.1	%		93.8	%		93.7	%

SCHEDULE OF SUPPLEMENTAL UNDERWRITING DATA–SIX MONTHS ENDED JUNE 30, 2011 (Unaudited)

(Expressed in thousands of United States Dollars)

	Property & Casualty															Life & Annuity			Corporate			Consolidated
	Insurance			Reinsurance			U.S. Specialty			Alterra at Lloyd’s			Total			Reinsurance
Period Segment Information:
Gross premiums written	$	212,527		$	623,774		$	169,836		$	184,253		$	1,190,390		$	1,365		$	—		$	1,191,755
Reinsurance premiums ceded		(99,248	)		(65,564	)		(60,953	)		(48,097	)		(273,862	)		(121	)		—			(273,983	)
Net premiums written	$	113,279		$	558,210		$	108,883		$	136,156		$	916,528		$	1,244		$	—		$	917,772
Earned premiums	$	196,240		$	459,113		$	147,017		$	128,535		$	930,905		$	1,365		$	—		$	932,270
Earned premiums ceded		(89,068	)		(32,500	)		(46,633	)		(35,120	)		(203,321	)		(121	)		—			(203,442	)
Net premiums earned		107,172			426,613			100,384			93,415			727,584			1,244			—			728,828
Net losses and loss expenses		(63,656	)		(312,477	)		(64,084	)		(75,322	)		(515,539	)		—			—			(515,539	)
Claims and policy benefits		—			—			—			—			—			(30,280	)		—			(30,280	)
Acquisition costs		177			(93,509	)		(17,710	)		(23,965	)		(135,007	)		(281	)		—			(135,288	)
General and administrative expenses		(18,684	)		(46,195	)		(18,686	)		(19,259	)		(102,824	)		(436	)		—			(103,260	)
Other income		951			548			—			380			1,879			(23	)		—			1,856
Underwriting income (loss)	$	25,960		$	(25,020	)	$	(96	)	$	(24,751	)	$	(23,907	)		n/a			—			n/a
Net investment income																	24,888			92,543			117,431
Net realized and unrealized gains (losses) on investments																	1,508			(26,100	)		(24,592	)
Net impairment losses recognized in earnings																				(1,382	)		(1,382	)
Corporate other income																				50			50
Interest expense																				(19,089	)		(19,089	)
Net foreign exchange losses																				(2,212	)		(2,212	)
Corporate general and administrative expenses																				(37,602	)		(37,602	)
(Loss) income before taxes																$	(3,380	)	$	6,208		$	(21,079	)
Loss ratio [a]		59.4	%		73.2	%		63.8	%		80.6	%		70.9	%
Acquisition cost ratio [b]		(0.2	%)		21.9	%		17.6	%		25.7	%		18.6	%
General and adminstrative expense ratio [c]		17.4	%		10.8	%		18.6	%		20.6	%		14.1	%
Combined ratio [d]		76.7	%		106.0	%		100.1	%		126.9	%		103.5	%

[a] The loss ratio is calculated by dividing net losses and loss expenses by net premiums earned.

[b] The acquisition cost ratio is calculated by dividing acquisitions costs by net premiums earned.

[c] The general and administrative expense ratio is calculated by dividing general and administrative expenses by net premiums earned.

[d] The combined ratio is calculated by dividing the sum of net losses and loss expenses, acquisition costs and general and administrative expenses by net premiums earned.

Percentage totals may not add due to rounding.

ALTERRA CAPITAL HOLDINGS LIMITED

SCHEDULE OF SUPPLEMENTAL UNDERWRITING DATA–THREE MONTHS ENDED JUNE 30, 2010 (Unaudited)

(Expressed in thousands of United States Dollars)

	Property & Casualty															Life & Annuity			Corporate			Consolidated
	Insurance [e]			Reinsurance			U.S. Specialty [e]			Alterra at Lloyd’s			Total			Reinsurance
Quarter Segment Information:
Gross premiums written	$	139,026		$	119,578		$	90,773		$	48,802		$	398,179		$	802		$	—		$	398,981
Reinsurance premiums ceded		(47,437	)		(13,348	)		(5,672	)		(13,233	)		(79,690	)		(25	)		—			(79,715	)
Net premiums written	$	91,589		$	106,230		$	85,101		$	35,569		$	318,489		$	777		$	—		$	319,266
Earned premiums	$	104,841		$	174,729		$	71,065		$	40,286		$	390,921		$	802		$	—		$	391,723
Earned premiums ceded		(45,211	)		(19,054	)		(27,053	)		(7,120	)		(98,438	)		(25	)		—			(98,463	)
Net premiums earned		59,630			155,675			44,012			33,166			292,483			777			—			293,260
Net losses and loss expenses		(38,296	)		(82,441	)		(26,683	)		(12,397	)		(159,817	)		—			—			(159,817	)
Claims and policy benefits		—			—			—			—			—			(13,943	)		—			(13,943	)
Acquisition costs		(821	)		(32,673	)		(9,111	)		(6,070	)		(48,675	)		(123	)		—			(48,798	)
General and administrative expenses		(8,132	)		(14,820	)		(6,355	)		(5,826	)		(35,133	)		(642	)		—			(35,775	)
Other income		97			155			—			173			425			—			—			425
Underwriting income	$	12,478		$	25,896		$	1,863		$	9,046		$	49,283			n/a			—			n/a
Net investment income																	12,420			40,857			53,277
Net realized and unrealized losses on investments																	(1,860	)		(12,926	)		(14,786	)
Net impairment losses recognized in earnings																				(300	)		(300	)
Corporate other income																				(150	)		(150	)
Interest expense																				(7,916	)		(7,916	)
Net foreign exchange gains																				634			634
Merger and acquisition expenses																				54,570			54,570
Corporate general and administrative expenses																				(14,874	)		(14,874	)
(Loss) income before taxes																$	(3,371	)	$	59,895		$	105,807
Loss ratio [a]		64.2	%		53.0	%		60.6	%		37.4	%		54.6	%
Acquisition cost ratio [b]		1.4	%		21.0	%		20.7	%		18.3	%		16.6	%
General and adminstrative expense ratio [c]		13.6	%		9.5	%		14.4	%		17.6	%		12.0	%
Combined ratio [d]		79.2	%		83.5	%		95.8	%		73.2	%		83.3	%

SCHEDULE OF SUPPLEMENTAL UNDERWRITING DATA–SIX MONTHS ENDED JUNE 30, 2010 (Unaudited)

(Expressed in thousands of United States Dollars)

	Property & Casualty															Life & Annuity			Corporate			Consolidated
	Insurance [e]			Reinsurance			U.S. Specialty [e]			Alterra at Lloyd’s			Total			Reinsurance
Period Segment Information:
Gross premiums written	$	208,432		$	274,429		$	164,675		$	120,919		$	768,455		$	1,665		$	—		$	770,120
Reinsurance premiums ceded		(87,088	)		(58,699	)		(53,824	)		(33,192	)		(232,803	)		(132	)		—			(232,935	)
Net premiums written	$	121,344		$	215,730		$	110,851		$	87,727		$	535,652		$	1,533		$	—		$	537,185
Earned premiums	$	206,587		$	270,472		$	140,250		$	77,746		$	695,055		$	1,665		$	—		$	696,720
Earned premiums ceded		(94,041	)		(35,012	)		(65,446	)		(14,589	)		(209,088	)		(132	)		—			(209,220	)
Net premiums earned		112,546			235,460			74,804			63,157			485,967			1,533			—			487,500
Net losses and loss expenses		(77,742	)		(132,506	)		(45,879	)		(28,655	)		(284,782	)		—			—			(284,782	)
Claims and policy benefits		—			—			—			—			—			(31,602	)		—			(31,602	)
Acquisition costs		(504	)		(47,639	)		(12,793	)		(11,837	)		(72,773	)		(269	)		—			(73,042	)
General and administrative expenses		(14,979	)		(23,779	)		(13,877	)		(8,539	)		(61,174	)		(1,299	)		—			(62,473	)
Other income		91			155			—			351			597			(28	)		—			569
Underwriting income	$	19,412		$	31,691		$	2,255		$	14,477		$	67,835			n/a			—			n/a
Net investment income																	25,519			76,148			101,667
Net realized and unrealized gains (losses) on investments																	4,056			(12,420	)		(8,364	)
Net impairment losses recognized in earnings																				(720	)		(720	)
Corporate other income																				50			50
Interest expense																				(12,858	)		(12,858	)
Net foreign exchange gains																				3,086			3,086
Merger and acquisition expenses																				49,826			49,826
Corporate general and administrative expenses																				(24,704	)		(24,704	)
(Loss) income before taxes																$	(2,090	)	$	78,408		$	144,153
Loss ratio [a]		69.1	%		56.3	%		61.3	%		45.4	%		58.6	%
Acquisition cost ratio [b]		0.4	%		20.2	%		17.1	%		18.7	%		15.0	%
General and adminstrative expense ratio [c]		13.3	%		10.1	%		18.6	%		13.5	%		12.6	%
Combined ratio [d]		82.8	%		86.6	%		97.0	%		77.6	%		86.2	%

[a] The loss ratio is calculated by dividing net losses and loss expenses by net premiums earned. [b] The acquisition cost ratio is calculated by dividing acquisitions costs by net premiums earned.

[c] The general and administrative expense ratio is calculated by dividing general and administrative expenses by net premiums earned.

[d] The combined ratio is calculated by dividing the sum of net losses and loss expenses, acquisition costs and general and administrative expenses by net premiums earned.

[e] Insurance and U.S. specialty segment amounts for the comparative 2010 periods have been reclassified to conform with the current period’s presentation.

Percentage totals may not add due to rounding.

ALTERRA CAPITAL HOLDINGS LIMITED

SCHEDULE OF SUPPLEMENTAL PREMIUM DATA–SIX MONTHS ENDED JUNE 30, 2011 (Unaudited)

(Expressed in thousands of United States Dollars)

		Six Months Ended June 30, 2011									Six Months Ended June 30, 2010
		Gross Premiums Written			Percentage of Total Gross Premiums Written			Movement on Prior Year Period			Gross Premiums Written			Percentage of Total Gross Premiums Written
Gross Premiums Written by Type of Risk:
Property & Casualty:
Insurance [a]:
Aviation	S	$	5,230			0.4	%		(41.7	%)	$	8,964			1.2	%
Excess Liability	L		68,777			5.8	%		6.4	%		64,634			8.4	%
Professional Liability	L		92,813			7.8	%		(2.0	%)		94,700			12.3	%
Property	S		45,707			3.8	%		13.9	%		40,134			5.2	%
			212,527			17.8	%		2.0	%		208,432			27.1	%
Reinsurance [b]:
Agriculture	S		29,498			2.5	%		(11.4	%)		33,293			4.3	%
Auto	S		70,265			5.9	%		n/m			66			0.0	%
Aviation	S		984			0.1	%		(92.6	%)		13,383			1.7	%
Credit/ Surety	S		25,982			2.2	%		n/m			(1,491	)		(0.2	%)
General Casualty	L		38,292			3.2	%		214.0	%		12,194			1.6	%
Marine & Energy	S		16,474			1.4	%		81.0	%		9,104			1.2	%
Medical Malpractice	L		28,805			2.4	%		(17.6	%)		34,968			4.5	%
Other	S		2,206			0.2	%		104.1	%		1,081			0.1	%
Professional Liability	L		99,146			8.3	%		125.7	%		43,924			5.7	%
Property	S		256,042			21.5	%		139.3	%		106,991			13.9	%
Whole Account	S/L		35,338			3.0	%		n/m			4,935			0.6	%
Workers’ Compensation	L		20,742			1.7	%		29.8	%		15,981			2.1	%
			623,774			52.3	%		127.3	%		274,429			35.6	%
U.S. Specialty [a]:
General Liability	L		36,270			3.0	%		(15.4	%)		42,852			5.6	%
Marine	S		42,498			3.6	%		24.8	%		34,059			4.4	%
Professional Liability	L		9,249			0.8	%		131.2	%		4,001			0.5	%
Property	S		81,819			6.9	%		(2.3	%)		83,763			10.9	%
			169,836			14.3	%		3.1	%		164,675			21.4	%
Alterra at Lloyd’s:
Accident & Health	S		21,924			1.8	%		12.8	%		19,432			2.5	%
Aviation	S		3,577			0.3	%		(48.7	%)		6,968			0.9	%
Financial Institutions	L		15,477			1.3	%		41.5	%		10,934			1.4	%
International Casualty	L		42,346			3.6	%		113.2	%		19,866			2.6	%
Professional Liability	L		18,388			1.5	%		66.3	%		11,057			1.4	%
Property	S		82,560			6.9	%		56.8	%		52,662			6.8	%
Surety	S		(19	)		0.0	%		n/m			—			0.0	%
			184,253			15.5	%		52.4	%		120,919			15.7	%
Aggregate Property & Casualty		$	1,190,390			99.9	%		54.9	%	$	768,455			99.8	%
Life & Annuity:
Annuity		$	607			0.1	%		n/m		$	6			0.0	%
Life			758			0.1	%		(54.3	%)		1,659			0.2	%
Aggregate Life & Annuity		$	1,365			0.1	%		(18.0	%)	$	1,665			0.2	%
Aggregate Property & Casualty and Life & Annuity		$	1,191,755			100.0	%		54.7	%	$	770,120			100.0	%
S = Short tail lines		$	702,416			59.0	%				$	410,877			53.5	%
L = Long tail lines			487,974			41.0	%					357,578			46.5	%
Aggregate Property & Casualty		$	1,190,390								$	768,455

[a] Insurance and U.S. specialty segment amounts for the comparative 2010 periods have been reclassified to conform with the current period’s presentation.

[b] Includes the results of Harbor Point Limited from May 12, 2010.

Percentage totals may not add due to rounding.

n/m Not meaningful.

ALTERRA CAPITAL HOLDINGS LIMITED

NON-GAAP FINANCIAL MEASURE RECONCILIATIONS (Unaudited)

Net Operating Income and Net Operating Income per Diluted Share

(Expressed in thousands of United States Dollars, except per share and share amounts)

	Three Months Ended June 30,					Six Months Ended June 30,
	2011		2010			2011			2010
Net income (loss) before tax	$	28,308	$	105,807		$	(21,079	)	$	144,153
Net realized and unrealized losses on non-hedge fund investments, before tax [a]		4,249		11,763			27,263			13,131
Foreign exchange losses (gains), before tax		3,090		(634	)		2,212			(3,086	)
Merger and acquisition expenses, before tax		—		(54,570	)		—			(49,826	)
Net operating income before tax	$	35,647	$	62,366		$	8,396		$	104,372
Net income (loss)	$	32,635	$	103,447		$	(14,052	)	$	139,828
Net realized and unrealized losses on non-hedge fund investments, net of tax [a]		4,711		11,540			27,313			12,776
Foreign exchange losses (gains), net of tax		2,212		(298	)		1,572			(1,978	)
Merger and acquisition expenses, net of tax		—		(55,915	)		—			(51,171	)
Net operating income	$	39,558	$	58,774		$	14,833		$	99,455
Net income (loss) per diluted share	$	0.30	$	1.13		$	(0.13	)	$	1.88
Net realized and unrealized losses on non-hedge fund investments, net of tax [a]		0.04		0.12			0.26			0.17
Foreign exchange losses (gains), net of tax		0.02		—			0.01			(0.03	)
Merger and acquisition expenses, net of tax		—		(0.61	)		—			(0.69	)
Net operating income per diluted share	$	0.37	$	0.64		$	0.14		$	1.33
Weighted average shares outstanding - basic		105,604,786		91,042,301			106,385,007			73,779,447
Weighted average shares outstanding - diluted		107,111,909		91,661,430			106,385,007			74,524,919

[a] Net realized and unrealized (gains) losses on non-hedge fund investments includes realized and unrealized (gains) losses on trading securities, realized (gains) losses on available for sale securities, net impairment losses recognized in earnings, earnings from equity method investments and changes in fair value of investment derivatives, catastrophe bonds and structured deposits.

Per share totals may not add due to rounding.

Annualized Net Operating Return on Average Shareholders’ Equity

(Expressed in thousands of United States Dollars)

	Three Months Ended June 30,						Six Months Ended June 30,
	2011			2010			2011			2010
Net income (loss)	$	32,635		$	103,447		$	(14,052	)	$	139,828
Annualized net income (loss)		130,540			413,788			(28,104	)		279,656
Net operating income	$	39,558		$	58,774		$	14,833		$	99,455
Annualized net operating income		158,232			235,096			29,666			198,910
Average shareholders’ equity [b]	$	2,758,154		$	2,321,687		$	2,789,443		$	1,955,281
Annualized return on average shareholders’ equity		4.7	%		17.8	%		(1.0	%)		14.3	%
Annualized net operating return on average shareholders’ equity		5.7	%		10.1	%		1.1	%		10.2	%

[b] Average shareholders’ equity is computed as the average of the quarterly average shareholders’ equity balances. The averages for the three and six months ended June 30, 2010 have been weighted to include Harbor Point Limited from May 12, 2010.

Diluted Tangible Book Value Per Share

(Expressed in thousands of United States Dollars, except per share and share amounts)

	June 30, 2011		December 31, 2010
Shareholders’ equity	$	2,793,081	$	2,918,270
Goodwill and intangible assets		58,094		59,076
Tangible book value	$	2,734,987	$	2,859,194
Diluted shares outstanding		107,500,490		112,290,968
Diluted tangible book value per share	$	25.44	$	25.46

ALTERRA CAPITAL HOLDINGS LIMITED

SCHEDULE OF SUPPLEMENTAL INVESTMENT DATA– JUNE 30, 2011 (Unaudited)

(Expressed in thousands of United States Dollars)

Type of Investment	As of June 30, 2011		Investment Distribution			As of December 31, 2010		Investment Distribution
Cash and cash equivalents	$	987,057		12.4	%	$	905,606		11.5	%
U.S. government and agencies		744,465		9.4	%		995,546		12.7	%
Non-U.S. governments		168,290		2.1	%		79,111		1.0	%
Corporate securities		2,743,228		34.5	%		2,735,366		34.8	%
Municipal securities		218,370		2.7	%		238,014		3.0	%
Asset-backed securities		203,598		2.6	%		86,937		1.1	%
Residential mortgage-backed securities		1,159,922		14.6	%		1,168,390		14.9	%
Commercial mortgage-backed securities		383,920		4.8	%		334,151		4.3	%
Fixed maturities at fair value	$	5,621,793		70.7	%	$	5,637,515		71.7	%
U.S. government and agencies	$	29,694		0.4	%	$	29,687		0.4	%
Non-U.S. governments		584,039		7.3	%		538,274		6.8	%
Corporate securities		378,958		4.8	%		371,143		4.7	%
Asset-backed securities		1,000		0.0	%		1,000		0.0	%
Fixed maturities at amortized cost	$	993,691		12.5	%	$	940,104		12.0	%
Other investments	$	350,193		4.4	%	$	378,128		4.8	%
Total invested assets	$	7,952,734		100.0	%	$	7,861,353		100.0	%
Credit Rating	As of June  30, 2011		Ratings Distribution			As of December 31, 2010		Ratings Distribution
U.S. government and agencies [a]	$	1,866,591		28.2	%	$	2,060,116		31.3	%
AAA		1,199,207		18.1	%		1,076,680		16.4	%
AA		656,570		9.9	%		657,867		10.0	%
A		1,406,390		21.3	%		1,353,945		20.6	%
BBB		266,150		4.0	%		226,849		3.4	%
BB		24,165		0.4	%		32,021		0.5	%
B		106,444		1.6	%		138,703		2.1	%
CCC or lower		46,565		0.7	%		45,469		0.7	%
Not rated		49,711		0.8	%		45,865		0.7	%
Fixed maturities at fair value	$	5,621,793		85.0	%	$	5,637,515		85.7	%
U.S. government and agencies	$	29,694		0.4	%	$	29,687		0.5	%
AAA		696,439		10.5	%		641,437		9.8	%
AA		117,851		1.8	%		113,140		1.7	%
A		135,249		2.0	%		141,683		2.2	%
BBB		14,458		0.2	%		12,744		0.2	%
Not rated		—		—			1,413		0.0	%
Fixed maturities at amortized cost	$	993,691		15.0	%	$	940,104		14.3	%
Total fixed maturities	$	6,615,484		100.0	%	$	6,577,619		100.0	%

[a] Included within U.S. government and agencies are agency-issued residential mortgage-backed securities with a fair value of $1,122,126 (December 31, 2010: $1,064,570)

Percentage totals may not add due to rounding.

	Three Months Ended June  30,						Six Months Ended June 30,
	2011			2010			2011			2010
Net investment income	$	59,665		$	53,277		$	117,431		$	101,667
Realized and unrealized gains (losses) on trading fixed maturities		1,432			(1,284	)		(395	)		(1,466	)
Net realized gains on available for sale fixed maturities		54			2,425			3,423			3,263
(Decrease) increase in fair value of hedge funds		(1,878	)		(3,323	)		1,289			4,048
Decrease in fair value of non-hedge fund other investments		(5,382	)		(12,604	)		(28,909	)		(14,209	)
Net realized and unrealized losses on investments	$	(5,774	)	$	(14,786	)	$	(24,592	)	$	(8,364	)
Net impairment losses recognized in earnings	$	(353	)	$	(300	)	$	(1,382	)	$	(720	)

Contacts

Susan Spivak Bernstein, 1-212-898-6640

Senior Vice President

susan.spivak@alterra-bm.com

or

Kekst and Company

Peter Hill or Melissa Sheer, 1-212-521-4800

peter-hill@kekst.com / melissa-sheer@kekst.com