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                                  Exhibit 5.1

                                      -36-




                                                                December 1, 1995


Teradyne, Inc.
321 Harrison Avenue
Boston, Massachusetts 02118

      Re:     Registration Statement on Form S-8 Relating to the
              Megatest Corporation 1990 Stock Option Plan
              and the Megatest Corporation Director Stock Option
              Plan (collectively, the "Plans")     
              --------------------------------

Ladies and Gentlemen:

      Reference is made to the  above-captioned  Registration  Statement on Form
S-8 (the "Registration  Statement") filed by Teradyne, Inc. (the "Company") with
the  Securities  and Exchange  Commission  under the  Securities Act of 1933, as
amended,  relating to an aggregate of 603,401 shares of Common  Stock, $.125 par
value, of the Company (the "Shares").

      We are counsel to the Company and are familiar with the proceedings of its
stockholders  and Board of  Directors.  We have  examined  original or certified
copies of the Company's certificate of incorporation,  as amended, the Company's
by-laws,  as amended,  the corporate  records of the Company to the date hereof,
and such other certificates,  documents, records and materials as we have deemed
necessary in connection with this opinion letter.

      We are members only of the Bar of the  Commonwealth of  Massachusetts  and
are  not  experts  in,  and  express  no  opinion  regarding,  the  laws  of any
jurisdiction  other than the Commonwealth of Massachusetts and the United States
of America, and the General Corporation Law of the State of Delaware.

      Based upon and subject to the  foregoing,  we are of the opinion  that the
Shares issued or proposed to be issued by the Company pursuant to the Plans will
be, upon receipt of the consideration provided for in the Plans, validly issued,
fully paid and  nonassessable  after issuance of such Shares in accordance  with
the terms of the Plans.

      We hereby  consent  to the filing of this  opinion  as Exhibit  5.1 to the
Registration Statement.

                               Very truly yours,


                               /s/ Testa, Hurwitz & Thibeault
                               TESTA, HURWITZ & THIBEAULT