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                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                         AMENDMENT NO. 1 TO SCHEDULE 13D
                    Under the Securities Exchange Act of 1934

                                    UST CORP.
                         (formerly Walden Bancorp, Inc.)
                                (Name of Issuer)

                         Common Stock ($.625 par value)
                         (Title of Class of Securities)

                                    902900109
                                 (CUSIP Number)

                            Jill D. Cookman, Attorney
                            W. R. Berkley Corporation
                                165 Mason Street
                            Greenwich, CT 06836-2518
                                 (203) 629-3000

(Name, Address and Telephone number of Person Authorized to Receive Notices and
                                Communications)

                                 January 3, 1997
             (Date of Event which Requires Filing of this Statement)

         If the filing person has previously filed a statement of Schedule 13G
to report the acquisition which is the subject of this Schedule 13D, and is
filing this schedule because of Rule 13d-1(b)(3) or (4), check the following 
box.

         Check the following box if a fee is being paid with this statement . (A
fee is not required only if the reporting person: (1) has a previous statement
on file reporting beneficial ownership of more than five percent of the class of
securities described in Item 1; and (2) has filed no amendment subsequent
thereto reporting beneficial ownership of less than five percent of such class.

         Note: Six copies of this statement, including all exhibits, should be
filed with the Commission. See Rule 13d-1(a) for other parties to whom copies
are to be sent.

         *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 disclosures provided in a prior cover page.

         The information required on 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 
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liabilities of that section of the Act but shall be subject to all other
provisions of the Act (however, see the Notes).

CUSIP No. 902900109

(2)      Check the Appropriate Box if a Member of a Group. (See Instructions):
         (a)
         (b) X
- ------------------------------------------------------------------------------
(3)      SEC use Only

- ------------------------------------------------------------------------------
(4)      Source of Funds (See Instructions):  AF

- ------------------------------------------------------------------------------
(5)      Check if Disclosure of Legal Proceedings is Required Pursuant to Items
         2(d) or 2(e):

- ------------------------------------------------------------------------------
(6)      Citizenship or Place of Organization:  Delaware

- ------------------------------------------------------------------------------
Number of Shares              (7)  Sole Voting Power: 557,650
Beneficially Owned            (8)  Shared Voting Power:  0
by Each Reporting             (9)  Sole Dispositive Power:  557,650
Person With                   (10)  Shared Dispositive Power:  0

- ------------------------------------------------------------------------------
(11)     Aggregate Amount Beneficially Owned by Each Reporting Person:  557,650

- ------------------------------------------------------------------------------
(12)     Check if the Aggregate Amount in Row (11) Excludes Certain Shares
         (See Instructions):

- ------------------------------------------------------------------------------
(13)     Percent of Class Represented by Amount in Row (11):  3.11%

- ------------------------------------------------------------------------------
(14)     Type of Reporting Person (See Instructions):  HC

- ------------------------------------------------------------------------------
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Item 1.           Security and Issuer.

         This Statement relates to the common stock, par value $.625 per share
(the "Shares"), of UST Corp., a Massachusetts corporation (formerly Walden
Bancorp, Inc.) (the "Issuer"). According to the Issuer's Form 10-Q for the
period ended September 30, 1996, the Issuer's principal executive offices are
located at 40 Court Street, Boston, MA 02108. The filing person owned 293,500
shares of Walden Bancorp, Inc., which was acquired in January 1997 by Issuer,
resulting in a tax-free exchange of 1.9 shares of the Issuer for each share of
Walden Bancorp, Inc. common stock.

Item 2.           Identity and Background.

         (a)      W. R. Berkley Corporation ("Berkley")

         (b)      165 Mason Street, Greenwich, Connecticut 06830.

         (c)      Berkley is a Delaware corporation incorporated January 20,
                  1970.

Berkley is a holding company which, through its subsidiaries, operates in all
segments of the insurance industry. The units are grouped for management
purposes in five segments according to market served: regional property casualty
insurance; reinsurance; specialty insurance; alternative markets; and
International.

                  Reference is made to the current list of the directors and
executive officers of Berkley appearing as Item 10 on pp. 52-54 of Berkley's
Form 10-K attached hereto as Exhibit A. The following changes are not reflected
in the Form 10-K attached:

                  On December 3, 1996 Scott M. Cunningham retired from the Board
of Directors of Berkley.

                  On May 21, 1996, the Board of Directors of Berkley elected H.
Raymond Lankford, Jr. to the office of Senior Vice President - Alternative
Markets Operations effective May 1,1996. Mr. Lankford was President of All
American Agency Facilities, Inc. ("All American"), a subsidiary of Berkley,
since 1991. He joined All American in 1990.
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                  On September 11, 1996, the Board of Directors of Berkley
elected Jennifer C. Smith to the office of Vice President - Medical Care
Management of Berkley effective August 19, 1996. Ms. Smith was also named to the
position of President of Berkley Care Network, Inc., a wholly-owned subsidiary
of Berkley. Previously, Ms. Smith was Vice President, Occupational Managed Care
of Aetna Life & Casualty. She joined Aetna Life & Casualty in 1987.

                  On December 3, 1996, the Board of Directors of Berkley elected
Robert P. Cole to the office of Vice President effective October 1,1996.
Previously, Mr. Cole served as Senior Vice President, Underwriting at
Christiania General Insurance Corporation of New York. He joined Christiania
General Insurance Corporation of New York in 1986.

                  Information as to persons owning 10% or more of the voting
securities of Berkley is incorporated herein by reference to "Principal
Stockholders" in Berkley's Proxy Statement for its Annual Meeting of
Shareholders, May 21, 1996, attached hereto as Exhibit B.

         (d) Neither Berkley nor any of the persons listed above have, during
the last 5 years, been convicted in a criminal proceeding (excluding traffic
violations).

         (e) For general information relating to litigation involving the
holding company system, reference is made to Note (5), "Commitments, Litigation
and Contingent Liabilities", appearing on page 42 of Berkley's Annual Report on
Form 10-K for the year ended December 31, 1995 pursuant to the Securities
Exchange Act of 1934.

         (f) Berkley is incorporated in the State of Delaware. Each individual
referenced above is a citizen of the United States.

Item 3.           Source and Amount of Funds or Other Consideration.

         Berkley may be deemed to have beneficial ownership of 557,650 Shares. A
total 
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cash consideration of $3,531,375 was paid for the Walden Bancorp, Inc. shares by
ten of Berkley's wholly-owned subsidiaries as listed as follows:
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         Company                                                  Shares
                                                              
         Union Insurance Company                                  47,500
         Union Standard Insurance Company                         68,400
         Continental Western Insurance Company                    30,400
         Tri-State Insurance Company of Minnesota                 38,000
         Admiral Insurance Company                                89,300
         Admiral/Signet                                           38,000
         Admiral/FacRe                                            58,900
         Firemen's Insurance Company of Washington, D.C           38,000
         Carolina Casualty Insurance Company                      68,400
         Nautilus Insurance Company                               80,750
                                                                 -------

                                                  TOTAL          557,650


         No part of the consideration was obtained through a loan. The source of
the funds was the general working capital of each subsidiary.

Item 4.           Purpose of Transaction.

         These Shares were originally acquired from The One Bancorp which
subsequently merged into and became known as Walden Bancorp Inc., which is now
merged into Issuer and registered with the SEC. These Shares were acquired as
referred to in Item 3. Berkley has acquired the Shares of Issuer in the ordinary
course of business and the Shares were not acquired for the purpose of and do
not have the effect of changing or influencing the control of Issuer and were
not acquired in connection with or as a participant in any transactions having
such purpose or effect.

Item 5.           Interest in Securities of the Issuer.

         (a) Berkley may be deemed to have beneficial ownership of 557,650
shares of Issuer which amounts to a 3.11% ownership interest in Issuer.

         (b) Berkley, through its subsidiaries, has the sole power to vote or to
direct the vote of 557,650 shares of the Issuer. Berkley, through its
subsidiaries, has the sole power to dispose or to direct the disposition of
557,650 shares of the Issuer.
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         (c) The only transaction in the Stock which was effected during the
past sixty days by persons named in response to paragraph (a) was the exchange
of each share of Walden Bancorp, Inc. for 1.9 shares of Issuer. This exchange
resulted in the decrease in the percentage of shares of Issuer owned by Berkley
to below 5%.

         (d)      Not applicable.

         (e)      January 3, 1997.

Item 6.           Contracts, Arrangements, Understandings or Relationships With 
                     Respect to Securities of the Issuer.

         There are no contracts, arrangements, understandings or relationships
with respect to securities of Issuer.

Item 7.           Material to be Filed as Exhibits.

         Exhibit A - Berkley's Form 10-K for the year ended December 31, 1995.

         Exhibit B - Berkley's Proxy Statement for its Annual Meeting of 
                     Shareholders, May 21,1996.

Signature.

         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.

Date:  February 12, 1997

W. R. Berkley Corporation



By: /s/ James G. Shiel
    ---------------------------
Name:   James G. Shiel
Title:  Senior Vice President - Investments