Exhibit 5.1





                               April 1, 2003




KeySpan Corporation
One MetroTech Center
Brooklyn, New York 11201-3850

Ladies and Gentlemen:

         We have acted as counsel to KeySpan Corporation, a New York
corporation (the "Company"), in connection with the Registration Statement on
Form S-3 (the "462(b) Registration Statement") filed by the Company with the
Securities and Exchange Commission (the "Commission") pursuant to Rule 462(b)
under the Securities Act of 1933, as amended (the "Act"), in respect of the
Company's Registration Statement on Form S-3 (File No. 333-82230) (the "2002
Registration Statement", and together with the 462(b) Registration Statement,
the "Registration Statements"), which Rule 462(b) Registration Statement
relates to senior debt securities of the Company (the "Senior Debt
Securities"). The Senior Debt Securities may be issued and sold or delivered
from time to time as set forth in the Registration Statements, any amendments
thereto, the prospectus contained therein (the "Prospectus") and supplements
to the Prospectus, and pursuant to Rule 415 under the Act, for an aggregate
initial offering price not to exceed $25,000,000 plus the initial aggregate
offering price of securities remaining available for issuance and sale under
the 2002 Registration Statement.

         The Senior Debt Securities will be issued under an Indenture, dated
as of November 1, 2000, between the Company and JPMorgan Chase Bank, as
Trustee (the "Senior Trustee"), as supplemented by the First Supplemental
Indenture thereto dated March 6, 2002 (the "Senior Indenture").

         We have examined the Registration Statements and the Senior Indenture
and the form of Senior Debt Securities, which have been filed with the
Commission as exhibits to the 2002 Registration Statement. We have also
examined the originals, or duplicates or certified or conformed copies, of
such records, agreements, instruments and other documents and have made such
other and further investigations as we have deemed relevant and necessary in
connection with the opinions expressed herein. As to questions of fact
material to this opinion, we have relied upon certificates of public officials
and of officers and representatives of the Company.

         In rendering the opinion set forth below, we have assumed the
genuineness of all signatures, the legal capacity of natural persons, the
authenticity of all documents submitted to us as originals, the conformity to
original documents of all documents submitted to us as duplicates or certified
or conformed copies, and the authenticity of the originals of such latter
documents.



We also have assumed that the Senior Indenture is the valid and
legally binding obligation of the Senior Trustee.

         Based upon the foregoing, and subject to the qualifications and
limitations stated herein, we are of the opinion that, assuming (a) the taking
of all necessary corporate action by the Board of Directors of the Company, a
duly constituted and acting committee of such Board or duly authorized
officers of the Company (such Board of Directors, committee or authorized
officers being referred to herein as the "Board") to approve (1) the issuance
and terms of the Senior Debt Securities and (2) the terms of the offering
thereof and related matters and (b) the due execution, authentication,
issuance and delivery of such Senior Debt Securities, upon payment of the
consideration therefor provided for in the applicable definitive purchase,
underwriting or similar agreement approved by the Board and otherwise in
accordance with the provisions of the Senior Indenture and such agreement,
such Senior Debt Securities will constitute valid and legally binding
obligations of the Company enforceable against the Company in accordance with
their terms.

         Our opinion set forth above is subject to the effects of (i)
bankruptcy, insolvency, fraudulent conveyance, reorganization, moratorium and
other similar laws relating to or affecting creditors' rights generally, (ii)
general equitable principles (whether considered in a proceeding in equity or
at law) and (iii) an implied covenant of good faith and fair dealing.

         We are members of the Bar of the State of New York, and we do not
express any opinion herein concerning any law other than the law of the State
of New York and the federal law of the United States.

         We hereby consent to the filing of this opinion letter as Exhibit 5.1
to the 462(b) Registration Statement and to the use of our name under the
caption "Legal Opinions" in the Prospectus incorporated by reference in the
462(b) Registration Statement.





                                           Very truly yours,

                                           /s/ Simpson Thacher & Bartlett

                                           SIMPSON THACHER & BARTLETT