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                                                                     EXHIBIT 5.1




                                December 21, 1998

MASSBANK Corp.
123 Haven Street
Reading, MA 01867

         Re:      Registration Statement on Form S-8
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Ladies and Gentlemen:

     This opinion is delivered in our capacity as counsel to MASSBANK Corp., a
Delaware corporation (the "Company"), in connection with the preparation and
filing with the Securities and Exchange Commission under the Securities Act of
1933, as amended (the "Act"), of a Registration Statement on Form S-8 (the
"Registration Statement") relating to an additional 170,000 shares (the
"Shares") of the Company's common stock, par value $1.00 per share ("Common
Stock"), which the Company may issue pursuant to the Company's Amended and
Restated 1994 Stock Incentive Plan (the "Plan").

     As counsel for the Company, we have examined copies of the Plan, the
Company's Restated Certificate of Incorporation and the Company's By-laws, each
as presently in effect, and such records, certificates and other documents of
the Company as we have deemed necessary or appropriate for the purposes of this
opinion.

     We are attorneys admitted to practice in The Commonwealth of Massachusetts.
We express no opinion concerning the laws of any jurisdiction other than the
laws of the United States of America, The Commonwealth of Massachusetts and the
General Corporation Law of the State of Delaware.

     Based on the foregoing, we are of the opinion that upon the issuance and
delivery of the Shares against payment therefor in accordance with the terms of
the Plan, the Shares will be legally issued, fully paid and non-assessable
shares of the Company's Common Stock.

     The foregoing assumes all requisite steps will be taken to comply with the
requirements of the Act and applicable requirements of state laws regulating the
offer and sale of securities.

     We hereby consent to the inclusion of this opinion as an exhibit to the
Registration Statement.

                                                Very truly yours,


                                                /s/ Goodwin, Procter & Hoar LLP

                                                GOODWIN, PROCTER & HOAR  LLP