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                                                                EXHIBIT 5.1


                               October 7, 1996

Ferrofluidics Corporation
40 Simon Street
Nashua, New Hampshire 03061

Ladies and Gentlemen:

        This opinion is furnished in connection with the filing of a
registration statement on Form S-8 (the "Registration Statement"), pursuant to
the Securities Act of 1933, as amended (the "Securities Act"), relating to
1,042,737 shares of common stock, par value $.004 per share (the "Shares"), of
Ferrofluidics Corporation, a Massachusetts corporation (the "Company").

        In connection with rendering this opinion, we have examined the
Restated Articles of Organization, as amended from time to time and the Amended 
and Restated By-laws of the Company; such records of the corporate proceedings
of the Company as we deemed material; the Registration Statement; such
certificates, receipts, records and documents of the Company as we considered
necessary for the purposes of this opinion; the Ferrofluidics Corporation
Amended and Restated 1994 Restricted Stock Plan (the "Restricted Plan"); the
Ferrofluidics Corporation Amended and Restated 1995 Non-Qualified Stock Option
Plan (the "Non-Qualified Plan"); and the Ferrofluidics Corporation Amended and
Restated 1995 Stock Option and Incentive Plan (the "Incentive Plan" and
together with the Restricted Plan and the Non-Qualified Plan, the "Plans").

        We are attorneys admitted to practice in the Commonwealth of
Massachusetts. We express no opinion concerning the laws of any jurisdictions
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 upon the foregoing, we are of the opinion that when the Shares
have been issued and paid for in accordance with the terms of the Plans, the
Shares will be legally issued, fully paid and nonassessable shares of common
stock, par value $.004 per share, of the Company.

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

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


                                                Very truly yours,

                                                /s/ GOODWIN, PROCTER & HOAR LLP

                                                GOODWIN, PROCTER & HOAR LLP