EXHIBIT 5           OPINION OF STEVEN L.  ZELKOWITZ AS TO THE LEGALITY
                    OF THE COMMON STOCK TO BE ISSUED


DECEMBER 12, 2000


KEYSPAN CORPORATION
D/B/A KEYSPAN ENERGY
ONE METROTECH CENTER
BROOKLYN, NY 11201


LADIES AND GENTLEMEN:

      AS SENIOR VICE PRESIDENT AND GENERAL COUNSEL OF KEYSPAN CORPORATION, A NEW
YORK CORPORATION (THE "COMPANY"),  I AM FAMILIAR WITH THE COMPANY'S REGISTRATION
STATEMENT  ON  FORM  S-3,   REGISTRATION  NUMBER  333-53657  (THE  "REGISTRATION
STATEMENT")  FILED  WITH  THE  SECURITIES  AND  EXCHANGE  COMMISSION  UNDER  THE
SECURITIES ACT OF 1933, AS AMENDED,  FOR THE REGISTRATION OF 5,000,000 SHARES OF
THE  COMPANY'S  COMMON  STOCK,  $0.01 PAR VALUE PER SHARE (THE "COMMON  STOCK"),
ISSUABLE  PURSUANT TO THE KEYSPAN  INVESTOR  PROGRAM (THE PROGRAM").  THE COMMON
STOCK REPRESENTS AUTHORIZED AND UNISSUED SHARES OF THE COMPANY'S COMMON STOCK.

I HAVE EXAMINED AND AM FAMILIAR WITH ORIGINALS OR COPIES, CERTIFIED OR OTHERWISE
IDENTIFIED TO MY SATISFACTION,  OF SUCH DOCUMENTS,  CORPORATE  RECORDS AND OTHER
INSTRUMENTS  AS I HAVE DEEMED  NECESSARY  OR  APPROPRIATE  FOR  PURPOSES OF THIS
OPINION.

      AS COUNSEL TO THE COMPANY, I HAVE PARTICIPATED IN AND AM FAMILIAR WITH THE
CORPORATE  PROCEEDINGS  OF  THE  COMPANY  RELATING  TO  THE  PREPARATION  OF THE
REGISTRATION STATEMENT, AS AMENDED, PROVIDING FOR THE REGISTRATION OF THE COMMON
STOCK FOR OFFERING AND SALE ON A DELAYED OR  CONTINUOUS  BASIS UNDER THE ACT AND
RULE 415 PROMULGATED THEREUNDER.

      In  connection  with  the  foregoing,  either  I or  individuals  under my
supervision  have researched such questions of law and examined the originals or
copies of the Registration  Statement and such corporate records,  agreements or
other  instruments of the Company and other  instruments and documents as I have
deemed  relevant and  necessary  in  connection  with the  opinions  hereinafter
expressed.  As to various  questions of fact material to such opinions,  I have,
where relevant facts were not independently

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established by me, relied upon statements of other officers of the Company, whom
I believe to be responsible.
           On the basis of the foregoing, I am of the opinion that:

                    o the Company has taken all  necessary  corporate  action to
                    authorize the issuance of the Common Stock;

                    o the shares of Common  Stock to be issued  pursuant  to the
                    Program are validly authorized and when issued and delivered
                    in accordance  with the terms of the Program,  the shares of
                    Common  Stock so issued will be validly  issued,  fully paid
                    and non-assessable.

           The  foregoing  opinion is  delivered to you in  connection  with the
Registration  Statement,  as  amended,  and may not be relied  upon by any other
person or for any other purpose.

      The opinions  expressed above are subject to the qualification that I am a
member of the Bar of the State of New York and such  opinions are limited to the
laws of the State of New York,  the laws of the United States of America and, to
the extent relevant to the opinion expressed above, the Business Corporation Law
of the State of New York.

      I hereby consent to the reference to me under the caption "Legal Opinions"
in the Prospectus  included in the  Registration  Statement and to the filing of
this  opinion  as  Exhibit 5 to the  Registration  Statement,  as  amended.  The
foregoing,  however, shall not constitute an admission by me that I am an expert
as provided for in Section 7 and 11 of the Act.

      I wish to call to your  attention  to the fact  that I am the  owner of or
have the option to acquire  approximately  277,500 shares of our common stock of
the Company.


                                                       Very truly yours,
                                                       /s/Steven L. Zelkowitz


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