EXHIBIT 5





                              THERMO TERRATECH INC.
                                 81 Wyman Street
                       Waltham, Massachusetts  02254-9046


                                  June 4, 1996


        Thermo TerraTech Inc.
        81 Wyman Street
        Waltham, Massachusetts  02254-9046


             Re:  Registration Statement on Form S-3

        Dear Sirs:

             I am General Counsel to Thermo TerraTech Inc., a Delaware
        corporation (the "Company"), and have acted as counsel in
        connection with the registration under the Securities Act of
        1933, as amended, on Form S-3 (the "Registration Statement"), of
        3,037,736 shares (the "Shares") of Common Stock, par value $.10
        per share, of the Company issuable upon conversion of $48,300,000
        principal amount of the Company's outstanding 4 5/8% Convertible
        Subordinated Debentures due 2003 (the "Debentures"). 

             I or a member of my staff have reviewed the corporate
        proceedings taken by the Company with respect to the
        authorization of the issuance of the Shares.  I or a member of my
        staff have also examined and relied upon originals or copies,
        certified or otherwise authenticated to my satisfaction, of all
        corporate records, documents, agreements or other instruments of
        the Company and have made all investigations of law and have
        discussed with the Company's representatives all questions of
        fact that I have deemed necessary or appropriate.

             Based upon and subject to the foregoing, I am of the opinion
        that:

             1.   The Company is a corporation duly organized, validly
        existing and in corporate good standing under the laws of the
        State of Delaware.

             2.   The issuance and sale of the Shares as contemplated in
        the Registration Statement have been duly authorized by the
        Company.
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             3.   The Shares, when issued upon conversion of the
        Debentures, will be validly issued, fully paid and nonassessable.

             I hereby consent to the filing of this opinion as Exhibit 5
        to the Registration Statement.

                                           Very truly yours,

                                           /s/Seth H. Hoogasian
                                           Seth H. Hoogasian
                                           General Counsel

 SHH/mj