1
 
                                                                     EXHIBIT 8.1
 
                 [LETTERHEAD OF WACHTELL, LIPTON, ROSEN & KATZ]
 
                                                                May 8, 1998
 
Essex County Gas Company
7 North Hunt Road
Amesbury, Massachusetts 01913
 
Ladies and Gentlemen:
 
     Reference is made to the Registration Statement on Form S-4 of Eastern
Enterprises ("Eastern"), a Massachusetts business trust, relating to the merger
of a wholly owned subsidiary of Eastern, with and into Essex County Gas Company
("Essex"). Any capitalized item used and not defined herein has the meaning
given to it in the Proxy Statement/Prospectus or the appendices thereto
(including the Agreement).
 
     We have prepared, solely for purposes of the Registration Statement on Form
S-4, the opinion set forth in the section entitled "THE MERGER (PROPOSAL
1) -- Federal Income Tax Consequences," which we confirm to be our opinion.
 
     We hereby consent to the filing of this opinion with the Securities and
Exchange Commission as an exhibit to the Registration Statement, and to the
reference to this opinion under the captions "SUMMARY -- THE MERGER AND THE
MERGER AGREEMENT -- Federal Income Tax Consequences," THE MERGER (PROPOSAL
1) -- Federal Income Tax Consequences," and "LEGAL MATTERS." In giving such
consent, we do not hereby admit that we are in the category of persons whose
consent is required under Section 7 of the Securities Act of 1933, as amended.
 
                                          Very truly yours,
 
                                          /s/  Wachtell, Lipton, Rosen & Katz