EXHIBIT 5
                                   UST CORP.
                                40 Court Street
                          Boston, Massachusetts 02108



                               December 23, 1997




UST Corp.
40 Court Street
Boston, Massachusetts  02108

      Re:  UST CORP. REGISTRATION STATEMENT ON FORM S-3
           RELATING TO 1,180,000 SHARES OF COMMON STOCK

Ladies and Gentlemen:

     This opinion is rendered to you in connection with the filing by UST
Corp., a Massachusetts corporation, (the "Corporation") of its Registration
Statement on Form S-3 (the "Registration Statement") with the Securities and
Exchange Commission, relating to the registration under the Securities Act of
1933 of 1,180,000 shares of the Corporation's Common Stock, par value $0.625
per share (the "Common Stock"), filed with Securities and Exchange Commission
on December 23, 1997, all of which Shares were issued, as unregistered shares,
on October 15, 1997 pursuant to the Terms of an Agreement and Plan of
Reorganization, dated as of August 12, 1997 (the "Agreement"), among the
Corporation, Firestone Financial Corp. ("Firestone") and each of the
stockholders of Firestone.

     In rendering this opinion as General Counsel of the Corporation, I and an
attorney in my office acting under my direction have participated with the
Corporation and its officers in the preparation, review and filing of the
Registration Statement and the related prospectus (the "Prospectus"), have
examined other corporate documents and records, have made such examination of
law, and have discussed with the officers and directors of the Corporation and
its subsidiaries such questions of fact as we have deemed necessary or
appropriate. We have also relied upon certificates and statements of such
officers and directors as to factual matters and have assumed the genuineness
of all signatures not known to us as well as the authenticity of all documents
submitted to us as copies.

     Subject to the foregoing and to the proposed additional proceedings being
taken as now contemplated prior to the issuance of the Common Stock, it is my
opinion that the Common Stock has been duly authorized and that the shares of 
Common Stock are validly issued, fully paid and non-assessable.

     I hereby consent to the filing of this opinion as an Exhibit to the
Registration Statement and to the use of my name in the Registration Statement
and the related Prospectus.


                               Very truly yours,



                               /s/Eric R. Fischer
                               -------------------------
                               Eric R. Fischer,
                               Executive Vice President,
                               General Counsel and Clerk