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                                                                    Exhibit 8(a)

                [Bingham, Dana & Gould LLP Form of Tax Opinion]

UST Corp.
40 Corn Street
Boston, Massachusetts 02108

Ladies and Gentlemen:

     This opinion is furnished to you pursuit to Section 6.02(d) of the
Affiliation Agreement and Plan of Reorganization dated as of August 30, 1996
(the "Affiliation Agreement"), between UST Corp., a Massachusetts corporation
("UST"), and Walden Bancorp, Inc., a Massachusetts corporation ("Walden").
Pursuit to the Affiliation Agreement and the related Agreement and Plan of
Merger dated as of August 30, 1996 (the "Plan of Merger"), among UST, Walden and
Mosaic Corp., a Massachusetts corporation ("Sub"), Sub, a newly incorporated
direct wholly-owned subsidiary of UST, will merge with and into Walden in a
transaction (the "Affiliation") in which the existing stockholders of Walden
will receive Common Stock, par value $0.625 per shoe, of UST ("UST Common
Stock") in exchange for their issued and outstanding shares of Common Stock, par
value $1.00 per shoe, of Walden ("Walden Common Stock"). You have requested our
opinion as to certain federal income tax consequences anticipated to follow from
implementation of the Affiliation Agreement and the Plan of Merger.

     For purposes of our opinion, we have examined and relied upon the originals
or copies, certified or otherwise identified to us to our satisfaction, of the
Affiliation Agreement, the Plan of Merger, the Joint Proxy Statement-Prospectus
dated November __, 1996 included in the Registration Statement on Form S-4 filed
by UST with the Securities and Exchange Commission in connection with the
Affiliation (Registration No. 333-____ ), and related documents (collectively,
the "Documents"). In that examination, we have assumed the genuineness of all
signatures, the authenticity and completeness of all documents purporting to be
originals (whether reviewed by us in original or copy form) and the conformity
to the originals of all documents purporting to be copies.

     As to certain factual matters, we have relied with your consent upon, and
our opinion is limited by, the representations of the various parties set forth
in the Documents and in the certificates from UST, Walden and Sub dated the date
hereof copies of which are attached hereto (the "Certificates"). Our opinion
assumes that all representations set forth in the Documents and in the
Certificates are true and correct in all material aspects as of the date hereof.
In addition, our opinion is limited solely to the provisions of the federal
Internal Revenue Code as now in effect (the "Code"), and the regulations,
rulings, and interpretations thereof in force as of this date and we assume no
obligation to advise you of changes in the law or fact that occur after the date
of this opinion.

     On the basis of and subject to the foregoing, and assuming due adoption and
implementation of the Affiliation Agreement and the Plan of Merger in accordance
with their respective terms (including satisfaction of all covenants and
conditions to the obligations of the parties without amendment or waiver
thereof) and consistent with the representations set out in the Documents and
Certificates, we are of the opinion that for federal income tax purposes, the
Affiliation will constitute a reorganization under Section 368 of the Code.


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Bank of Boston Corporation
_______________,1996
Page 2

     This opinion is being delivered solely to you for your use in connection
with the referenced transaction and for such other uses to which we have given
our prior written consent. It may not be relied upon by any other person or used
for any other purpose.

                            Very truly yours,



                            BINGHAM, DANA & GOULD LLP