AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON MAY 14, 1999
                                                      REGISTRATION NO. 333-78285
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                      SECURITIES AND EXCHANGE COMMISSION
                            WASHINGTON, D.C. 20549

                                AMENDMENT NO. 1
                                      TO
                                   FORM S-4
                            REGISTRATION STATEMENT
                                     UNDER
                          THE SECURITIES ACT OF 1933

                                     NSTAR
            (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)

 
 
                                                                             
          MASSACHUSETTS                             6719                                04-3466300 
  (STATE OR OTHER JURISDICTION                (PRIMARY STANDARD                      (I.R.S. EMPLOYER
OF INCORPORATION OR ORGANIZATION)     INDUSTRIAL CLASSIFICATION CODE NUMBER)      IDENTIFICATION NUMBER) 
 

                                C/O BEC ENERGY
                              800 BOYLSTON STREET
                          BOSTON, MASSACHUSETTS 02199
                                (617) 424-2000
               (Address, including zip code, and telephone number,
       including area code, of registrant's principal executive offices)

                            DOUGLAS S. HORAN, ESQ.
                                     NSTAR
                                C/O BEC ENERGY
                              800 BOYLSTON STREET
                          BOSTON, MASSACHUSETTS 02199
                                (617) 424-2000
               (Name, address, including zip code, and telephone
              number, including area code, of agent for service)
              --------------------------------------------------  
                 PLEASE SEND COPIES OF ALL COMMUNICATIONS TO:

       DAVID A. FINE, ESQ.                      WARREN J. INGBER, ESQ.        
           ROPES & GRAY                 LEBOEUF, LAMB, GREENE & MACRAE, L.L.P.
     ONE INTERNATIONAL PLACE                     125 WEST 55TH STREET         
 BOSTON, MASSACHUSETTS 02110-2624              NEW YORK, NY  10019-5389       
          (617) 951-7000                            (212) 424-8000            
       (617) 951-7050 (FAX)                      (212) 424-8500 (FAX)          

                    _____________________________________       
  APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE TO THE PUBLIC: As soon as
      practicable after the effectiveness of the Registration Statement.
   If the only securities being registered on this form are being offered in
         connection with the formation of a holding company and there
    is compliance with General Instruction G, check the following box. [_]

THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR DATES
AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT SHALL FILE
A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION STATEMENT
SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(a) OF THE
SECURITIES ACT OF 1933 OR UNTIL THIS REGISTRATION STATEMENT SHALL BECOME
EFFECTIVE ON SUCH DATE AS THE SECURITIES AND EXCHANGE COMMISSION, ACTING
PURSUANT TO SAID SECTION 8(a), MAY DETERMINE.
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                                    PART II

                    INFORMATION NOT REQUIRED IN PROSPECTUS

ITEM 20.  INDEMNIFICATION OF TRUSTEES AND OFFICERS.

     NSTAR's declaration of trust provides that, to the extent legally
permissible, each of NSTAR's trustees and officers shall be indemnified by the
trust estate against any loss, liability or expense, including amounts paid in
satisfaction of judgments, in compromise or as fines and penalties, and counsel
fees, imposed upon or reasonably incurred by such person in connection with the
defense or disposition of any action, suit or other proceeding ,whether civil or
criminal, in which such person may be involved or with which such person may be
threatened, while in office or thereafter, by reason of such person's being or
having been such a trustee or officer, except with respect to any matter as to
which such person shall have been adjudicated in such action, suit or proceeding
not to have acted in good faith in the reasonable belief that his or her action
was in the best interests of NSTAR; provided, however, that as to any matter
disposed of by a compromise payment by such trustee or officer, pursuant to a
consent decree or otherwise, no indemnification either for said payment or for
any other expenses shall be provided unless such compromise shall be approved as
in the best interests of NSTAR, after notice that it involves such
indemnification, (i) by a disinterested majority of the trustees then in office,
or (ii) by a majority of the disinterested trustees then in office, provided
that there has been obtained an opinion in writing of independent legal counsel
to the effect that such trustee or officer appears to have acted in good faith
in the reasonable belief that his or her action was in the best interests of
NSTAR, or (iii) by the vote, at a meeting duly called and held, of the holders
of a majority of the shares outstanding and entitled to vote thereon, exclusive
of any shares owned by any interested trustee or officer.

     In discharging his or her duties, a trustee or officer of NSTAR, when
acting in good faith, shall be fully protected in relying upon the books of
account of NSTAR or of another organization in which he or she serves as
contemplated by the indemnification provisions of the declaration of trust,
reports made to NSTAR or to such other organization by any of its officers or
employees or by counsel, accountants, appraisers or other experts or consultants
selected with reasonable care by the trustees or similar governing body of such
other organization, or upon other records of NSTAR or of such other
organization. The rights of indemnification provided in the declaration of trust
shall not be exclusive of or affect any other rights to which any trustee or
officer may be entitled and such rights shall inure to the benefit of his or her
successors, heirs, executors, administrators and other legal representatives. As
used in the relevant provisions of the declaration of trust, the terms "trustee"
and "officer" include persons who serve at the request of NSTAR as directors,
officers, or trustees of another organization in which NSTAR has any direct or
indirect interest as a shareholder, creditor or otherwise.

     Expenses, including counsel fees, reasonably incurred by any trustee or
officer with respect to the defense or disposition of any action, suit or
proceeding referred to in the indemnification provisions of the declaration of
trust may be advanced by NSTAR prior to the final disposition of such action,
suit or proceeding, upon receipt of an undertaking by or on behalf of the
recipient to repay such amount unless it is ultimately determined that he or she
is entitled to indemnification. Nothing contained in these provisions affects
any rights to indemnification to which NSTAR personnel other than trustees and
officers may be entitled by contract or otherwise under law. No trustee shall be
obligated to give any bond or other security for the performance of any of his
or her duties.


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ITEM 21.  EXHIBITS AND FINANCIAL STATEMENT SCHEDULES.

 
     EXHIBIT                 DESCRIPTION
     --------                -----------
     NUMBER
     ------

     2.1       Amended and Restated Agreement and Plan of Merger, dated as of
               December 5, 1998, amended and restated as of May 4, 1999, by and
               among BEC Energy, Commonwealth Energy System, NSTAR, BEC
               Acquisition LLC and CES Acquisition LLC (attached as Annex A to
               the Joint Proxy Statement/Prospectus, which forms part of this
               Registration Statement).

     3.1       Declaration of Trust of NSTAR (attached as Annex D to the Joint
               Proxy Statement/Prospectus, which forms part of this Registration
               Statement).

     3.2       Bylaws of NSTAR (attached as Annex E to the Joint Proxy
               Statement/Prospectus, which forms part of this Registration
               Statement).

     5         Opinion of Ropes & Gray

     8.1       Tax Opinion of Ropes & Gray

     8.2       Tax Opinion of LeBoeuf, Lamb, Greene & MacRae LLP

     10.1      Employment Agreement between Thomas J. May and NSTAR, dated as of
               May 11, 1999 (attached as Exhibit 7.16.1 to the merger
               agreement in Annex A to the Joint Proxy Statement/Prospectus,
               which forms part of this Registration Statement).

     10.2      Employment Agreement between Russell D. Wright and NSTAR, dated
               as of May 11, 1999 (attached as Exhibit 7.16.2 to the merger
               agreement in Annex A to the Joint Proxy Statement/Prospectus,
               which forms part of this Registration Statement).

     10.3      Change of Control Agreements between Thomas J. May and NSTAR,
               dated as of May 11, 1999.

     10.4      Change in Control Agreement between Russell D. Wright and NSTAR,
               dated as of May 11, 1999.

     23.1      Consent of PricewaterhouseCoopers LLP

     23.2      Consent of Arthur Andersen LLP

     23.3      Consent of Goldman Sachs & Co.

     23.4      Consent of SG Barr Devlin

     23.5      Consent of Ropes & Gray (included in Exhibit 8.1)

     23.6      Consent of LeBoeuf, Lamb, Greene & MacRae LLP (included in
               Exhibit 8.2)

     99.1      Proxy Card for BEC Energy

     99.2      Proxy Card for Commonwealth Energy System

                                     II-2

 
ITEM 22.   UNDERTAKINGS.

     The undersigned registrant hereby undertakes:

          (1)  To file, during any period in which offers or sales are being 
     made, a post-effective amendment to this registration statement:

               (i)   To include any prospectus required by section 10(a)(3) of
          the Securities Act of 1933;
             
               (ii)  To reflect in the prospectus any facts or events arising 
          after the effective date of the registration statement (or the most
          recent post-effective amendment thereof) which, individually or in the
          aggregate, represent a fundamental change in the information set forth
          in the registration statement. Notwithstanding the foregoing, any
          increase or decrease in volume of securities offered (if the total
          dollar value of securities offered would not exceed that which was
          registered) and any deviation from the low or high end of the
          estimated maximum offering range may be reflected in the form of
          prospectus filed with the Commission pursuant to Rule 424(b) if, in
          the aggregate, the changes in volume and price represent no more than
          a 20% change in the maximum aggregate offering price set forth in the
          "Calculation of Registration Fee" table in the effective registration
          statement.

               (iii) To include any material information with respect to the 
          plan of distribution not previously disclosed in the registration
          statement or any material change to such information in the 
          registration statement.

          (2)  That, for the purpose of determining any liability under the 
     Securities Act of 1933, each such post-effective amendment shall be deemed
     to be a new registration statement relating to the securities offered
     therein, and the offering of such securities at that time shall be deemed
     to be the initial bona fide offering thereof.

          (3)  To remove from registration by means of a post-effective 
     amendment any of the securities being registered which remain unsold at the
     termination of the offering.

          (4)  That prior to any public reoffering of the securities registered
     hereunder through use of a prospectus which is a part of this registration
     statement, by any person or party who is deemed to be an underwriter within
     the meaning of Rule 145(c), the issuer undertakes that such reoffering
     prospectus will contain the information called for by the applicable
     registration form with respect to reofferings by persons who may be deemed
     underwriters, in addition to the information called for by the other items
     on the applicable form;

          (5)  That every prospectus (i) that is filed pursuant to paragraph (1)
     immediately preceding, or (ii) that purports to meet the requirements of
     Section 10(a)(3) of the Exchange Act and is used in connection with an
     offering of securities subject to Rule 415, will be filed as part of an
     amendment to the registration statement and will not be used until such
     amendment is effective, and that, for purposes of determining any liability
     under the Securities Act of 1933, each such post-effective amendment shall
     be deemed to be a new registration statement relating to the securities
     offered therein, and the offering of such securities at that time shall be
     deemed to be the initial bona fide offering thereof.

     The undersigned registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act of 1933, each filing of the
registrant's annual report pursuant to Section 13(a) or 15(d) of the Securities
Exchange Act of 1934 (and, where applicable, each filing of an employee benefit
plan's annual report pursuant to Section 15(d) of the Securities Exchange Act of
1934) that is incorporated by reference in the registration statement shall be
deemed to be a new registration statement relating to the securities offered
therein, and the offering of such securities at that time shall be deemed to be
the initial bona fide offering thereof.

     The undersigned registrant hereby undertakes to deliver or cause to be
delivered with the prospectus, to each person to whom the prospectus is sent or
given, the latest annual report to security holders that is incorporated by
reference in the prospectus and furnished pursuant to and meeting the
requirements of Rule 14a-3 or Rule 14c-3 under the Securities Exchange Act of
1934; and, where interim financial information required to be presented by
Article 3 of Regulation S-X is not set forth in the prospectus, to deliver, or
cause to be delivered, to each person to whom the prospectus is sent or given,
the latest quarterly report that is specifically incorporated by reference in
the prospectus so as to provide such interim financial information.

     Insofar as indemnification for liabilities arising under the Securities Act
of 1933 may be permitted to directors, officers and controlling persons of the
registrant pursuant to the foregoing provisions, or otherwise, the registrant
has been advised that in the opinion of the Securities and Exchange Commission
such indemnification is against public policy as expressed in the Act and is,
therefor, unenforceable. In the event that a claim for indemnification against
such liabilities (other than the payment by the registrant of expenses incurred
or paid by a director, officer or controlling person of the registrant in the
successful defense of any action, suit or proceeding) is asserted by such
director, officer or controlling person in connection with the securities being
registered, the registrant will, unless in the opinion of its counsel the matter
has been settled by controlling precedent, submit to a court of appropriate
jurisdiction the question of whether such indemnification by it is against
public policy as expressed in the Act and will be governed by the final
adjudication of such issue.

     The undersigned registrant hereby undertakes to supply by means of a post-
effective amendment all information concerning a transaction, and the company
being acquired involved therein, that was not the subject of and included in the
registration statement when it became effective.

     The undersigned registrant hereby undertakes to respond to requests
for information that is incorporated by reference into the prospectus pursuant
to Item 4, 10(b), 11, or 13 of this form, within one business day of receipt of
such request, and to send the incorporated documents by first class mail or
other equally prompt means. This includes 

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information contained in documents filed subsequent to the effective date of the
registration statement through the date of responding to the request.

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                                   SIGNATURES
 
   Pursuant to the requirements of the Securities Act of 1993, the registrant
has duly caused this registration statement to be signed on its behalf by the
undersigned, thereunto duly authorized, in the City of Boston, The Commonwealth
of Massachusetts, on the 14th day of May, 1999.
 
                                          NSTAR
 
                                          By: /s/ Thomas J. May
                                             ----------------------------------
                                          Name:  Thomas J. May
                                          Title: Chairman of the Board, Chief
                                                 Executive Officer and Treasurer
 
   Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities indicated on the 14th day of May, 1999.
 
   Signature                                 Title
   ---------                                 -----

   /s/ Thomas J. May
  -------------------------------            Chairman of the Board, Chief
   Thomas J. May                             Executive Officer and Treasurer
 
   /s/ Russell D. Wright
  -------------------------------            President, Chief Operating
   Russell D. Wright                         Officer, Clerk and Trustee
 
 
                                      II-5