Exhibit 5           Opinion of Steven L.  Zelkowitz  as to the  legality  of the
                    debt securities to be issued




                                                                October 13, 2000



KeySpan Corporation
One MetroTech Center
Brooklyn, NY 11201


Ladies and Gentlemen:

      I am Senior Vice President and General Counsel of KeySpan  Corporation,  a
New York  corporation  ("KeySpan"  or the  "Registrant").  I have  acted in such
capacity in connection with the  Registration  Statement on Form S-3, as amended
and  supplemented,  Registration No. 333-43768 (the  "Registration  Statement"),
filed by KeySpan with the Securities and Exchange  Commission (the "Commission")
under the  Securities  Act of 1933,  as  amended  (the  "Act")  relating  to the
issuance of one or more series of debt  securities  in the  aggregate  principal
amount of up to $1,650,000,000 (the "Debt Securities").

      Each series of Debt  Securities  will be issued  under an  Indenture  (the
"Indenture") dated as of November,  1, 2000, between KeySpan,  as issuer and The
Chase Manhattan Bank, as trustee (the  "Trustee").  A copy of such Indenture has
been filed as an Exhibit to the Registration Statement.

      As counsel to the Registrant,  I have participated in and am familiar with
the corporate  proceedings of the Registrant  relating to the preparation of the
Registration   Statement,   and  any  amendments  thereto,   providing  for  the
registration  of the Debt  Securities  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 the  Indenture  and such  corporate
records, agreements or other instruments of the Registrant 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 established by me,
relied upon statements of other officers of the Registrant, whom I believe to be
responsible.

      In rendering the opinions set forth below, I have assumed the  genuineness
of all signatures,  the legal capacity of natural  persons,  the authenticity of
all documents submitted to me as originals, the conformity to original documents
of all documents submitted to me as duplicates or certified or conformed copies,
and the  authenticity  of the  originals of such latter  documents.  I also have
assumed that at the time of execution, authentication,  issuance and delivery of
the Debt Securities, the Indenture

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will have been duly  executed and delivered by KeySpan and will be the valid and
legally binding obligation of the Trustee.

      Based  upon  the  foregoing,   and  subject  to  the   qualifications  and
limitations  stated herein, I am of the opinion that assuming (a) receipt of all
governmental  approvals  required in  connection  with the  issuance of the Debt
Securities,  (b) the taking of all  necessary  corporate  action to approve  the
issuance and terms of the Debt Securities, the terms of the offering thereof and
related  matters by the Board of Directors of KeySpan,  a duly  constituted  and
acting  committee  of such Board or duly  authorized  officers of KeySpan  (such
Board of Directors,  committee or authorized officers being hereinafter referred
to as the "KeySpan Board") and (c) the due execution,  authentication,  issuance
and delivery of the Debt Securities,  upon payment of the consideration therefor
provided for in the  applicable  definitive  purchase,  underwriting  or similar
agreement  approved by the KeySpan Board and  otherwise in  accordance  with the
provisions of the  Indenture,  the Debt  Securities  will  constitute  valid and
legally binding obligations of KeySpan enforceable against KeySpan in accordance
with their  terms,  except as may be limited by equitable  principles  which may
limit  the  availability  of  certain  equitable   remedies  (such  as  specific
performance)  in  certain  instances,  and  applicable  bankruptcy,  insolvency,
fraudulent conveyance, reorganization,  moratorium or similar laws related to or
affecting creditors' rights generally.

      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(a) to the  Registration  Statement.  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.

                                                      Very truly yours,


                                                      /s/ Steven L. Zelkowitz
                                                      -----------------------


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