UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

Amendment No.: 0*

Name of Issuer: SIX FLAGS ENTERTAINMENT CORPORATION

Title of Class of Securities: Common Stock

CUSIP Number: 83001A102

Date of Event Which Requires Filing of this Statement: 12/31/2018

Check the appropriate box to designate the rule pursuant to which this
Schedule is filed.

[X] Rule 13d-1(b) [ ] Rule 13d-1(c) [ ] Rule 13d-1(d)

* The remainder of this cover page shall be filled out for a reporting
person's initial filing on this form with respect to the subject class
of securities, and for any subsequent amendment containing information
which would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not
be deemed to be "filed" for the purpose of Section 18 of the Securities
Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of
that section of the Act but shall be subject to all other provisions of
the Act (however, see the Notes).

CUSIP No.: 83001A102

1. NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE
PERSON Janus Henderson Group plc EIN #00-0000000

2. CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP a. ___ b. ___ 3. SEC
USE ONLY

4. CITIZENSHIP OR PLACE OF ORGANIZATION Jersey, Channel Islands

NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:

5. SOLE VOTING POWER 0**

6. SHARED VOTING POWER 4,529,297**

7. SOLE DISPOSITIVE POWER 0**

8. SHARED DISPOSITIVE POWER 4,529,297**

9. AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
4,529,297**

10. CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES
Not applicable

11. PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 5.4%**

12. TYPE OF REPORTING PERSON IA, HC

** See Item 4 of this filing

Item 1. (a). Name of Issuer: SIX FLAGS ENTERTAINMENT CORPORATION ("Six
Flags")

(b). Address of Issuer's Principal Executive Offices:

924 Avenue J East Grand Prarie, TX 75050

Item 2. (a).-(c). Name, Principal Business Address, and Citizenship of
Persons Filing:

(1) Janus Henderson Group plc 201 Bishopsgate EC2M 3AE, United Kingdom
Citizenship: Jersey, Channel Islands

(d). Title of Class of Securities: Common Stock

(e). CUSIP Number: 83001A102

Item 3.

This statement is filed pursuant to Rule 13d-1 (b) or 13d-2(b) and the
person filing, Janus Henderson Group plc ("Janus Henderson") is a parent
holding company/control person in accordance with Section
240.13d-1(b)(ii)(G). See Item 4 for additional information.

Item 4. Ownership

The information in items 1 and 5 through 11 on the cover page(s) on
Schedule 13G is hereby incorporated by reference.

Janus Henderson has an indirect 97.11% ownership stake in Intech
Investment Management LLC ("Intech") and a 100% ownership stake in Janus
Capital Management LLC ("Janus Capital"), Janus Capital International
Limited ("JCIL"), Perkins Investment Management LLC ("Perkins"), Geneva
Capital Management LLC ("Geneva"), Henderson Global Investors Limited
("HGIL") and Janus Henderson Global Investors Australia Institutional
Funds Management Limited ("JHGIAIFML"), (each an "Asset Manager" and
collectively as the "Asset Managers"). Due to the above ownership
structure, holdings for the Asset Managers are aggregated for purposes
of this filing. Each Asset Manager is an investment adviser registered
or authorized in its relevant jurisdiction and each furnishing
investment advice to various fund, individual and/or institutional
clients (collectively referred to herein as "Managed Portfolios").

As a result of its role as investment adviser or sub-adviser to the
Managed Portfolios, Janus Capital may be deemed to be the beneficial
owner of 4,422,727 shares or 5.3% of the shares outstanding of Six Flags
Common Stock held by such Managed Portfolios. However, Janus Capital
does not have the right to receive any dividends from, or the proceeds
from the sale of, the securities held in the Managed Portfolios and
disclaims any ownership associated with such rights.

As a result of its role as investment adviser or sub-adviser to the
Managed Portfolios, JCIL may be deemed to be the beneficial owner of
98,682 shares or 0.1% of the shares outstanding of Six Flags Common
Stock held by such Managed Portfolios. However, Janus JCIL does not have
the right to receive any dividends from, or the proceeds from the sale
of, the securities held in the Managed Portfolios and disclaims any
ownership associated with such rights.

As a result of its role as investment adviser or sub-adviser to the
Managed Portfolios, Intech may be deemed to be the beneficial owner of
7,888 shares or 0.0% of the shares outstanding of Six Flags Common Stock
held by such Managed Portfolios. However, Intech does not have the right
to receive any dividends from, or the proceeds from the sale of, the
securities held in the Managed Portfolios and disclaims any ownership
associated with such rights.

Item 5. Ownership of Five Percent or Less of a Class

Not applicable.

Item 6. Ownership of More than Five Percent on Behalf of Another Person

The Managed Portfolios, set forth in Item 4 above, have the right to
receive all dividends from, and the proceeds from the sale of, the
securities held in their respective accounts.

The interest of any one such person does not exceed 5% of the class of
securities.

These shares were acquired in the ordinary course of business, and not
with the purpose of changing or influencing control of the Issuer.

Item 7. Identification and Classification of the Subsidiary Which
Acquired the Security Being Reported on by the Parent Holding Company

Intech, Janus Capital, JCIL, Perkins, Geneva, HGIL and JHGIAIFML are
indirect subsidiaries of Janus Henderson and are registered investment
advisers furnishing investment advice to Managed Portfolios.

Item 8. Identification and Classification of Members of the Group

Not applicable.

Item 9. Notice of Dissolution of Group

Not applicable.

Item 10. Certification

By signing below I certify that, to the best of my knowledge and belief,
the securities referred to above were acquired in the ordinary course of
business and were not acquired for the purpose of and do not have the
effect of changing or influencing the control of the issuer of such
securities and were not acquired in connection with or as a participant
in any transaction having such purposes or effect.

SIGNATURES

After reasonable inquiry and to the best of my knowledge and belief, I
certify that the information set forth in this statement is true,
complete and correct.

JANUS HENDERSON GROUP PLC

By /s/ Bruce Koepfgen
Bruce Koepfgen, Head of North America
Date 2/12/2019