Ropes & Gray
                            One International Place
                        Boston, Massachusetts 02110-2624
                                 (617) 951-7000
                              FAX: (617) 951-7050
                                                                  Exhibit 5.1
                                     

                                February 7, 1997


BBN Corporation
150 CambridgePark Drive
Cambridge, MA 02140

Ladies and Gentlemen:

         This  opinion is  furnished to you in  connection  with a  registration
statement on Form S-8 (the "Registration Statement"),  filed with the Securities
and Exchange  Commission (the "Commission") under the Securities Act of 1933, as
amended,  for the registration of an additional  850,000 shares of common stock,
$1.00  par value  (the  "Shares  of  Common  Stock"),  of BBN  Corporation  (the
"Company"),  which  are to be  issued  pursuant  to  the  Company's  1986  Stock
Incentive Plan (the "1986 Plan").

         We have acted as counsel for the  Company in the past and are  familiar
with the action  taken by the  Company  in  connection  with the 1986 Plan.  For
purposes of this opinion we have  examined  the 1986 Plan and certain  corporate
records of the Company,  including its Restated  Articles of  Organization,  its
By-laws,  minutes of meetings of its Board of Directors  and  stockholders,  and
such other documents as we deemed appropriate.

         We have made such  examination of  Massachusetts  law as we have deemed
relevant for purposes of this opinion,  but have not made any review of the laws
of any other  state or  jurisdiction.  Accordingly,  this  opinion is limited to
Massachusetts law.

         Based upon and subject to the foregoing, we are of the opinion that the
Shares of Common Stock to be issued and sold from time to time by the Company in
accordance  with the  terms of the 1986 Plan  will be duly  authorized,  validly
issued, fully paid and non-assessable.

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

                                  Very truly yours,
                                  /s/Ropes & Gray
                                  Ropes & Gray


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