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                                                                    Exhibit 5.01


                             CHOATE, HALL & STEWART
                A PARTNERSHIP INCLUDING PROFESSIONAL CORPORATIONS
                                 EXCHANGE PLACE
                                 53 STATE STREET
                        BOSTON, MASSACHUSETTS 02109-2891
                            TELEPHONE (617) 248-5000
                            FACSIMILE (617) 248-4000


                                                  August 18, 2000


The Board of Directors
Natural MicroSystems Corporation
100 Crossing Boulevard
Framingham, Massachusetts 01702

         Re:   Registration Statement on Form S-3

Ladies and Gentlemen:

         We have acted as counsel to Natural MicroSystems Corporation (the
"Company") in connection with the preparation of a Registration Statement on
Form S-3 (the "Registration Statement") filed by the Company with the Securities
and Exchange Commission (the "Commission") on or about August 18, 2000 under the
Securities Act of 1933, as amended (the "Act").

         The Registration Statement relates to the issuance and sale from time
to time, pursuant to Rule 415 of the General Rules and Regulations promulgated
under the Act, of the following securities with an aggregate public offering
price of up to $500,000,000: (i) senior or subordinated debt securities, in one
or more series (the "Debt Securities"), which may be issued under an Indenture
(the "Indenture"), proposed to be entered into between the Company and a trustee
to be selected (the "Trustee"), a form of which has been filed as an exhibit to
the Registration Statement; (ii) shares of preferred stock, $0.01 par value per
share (the "Preferred Stock"), in one or more series, (iii) shares of common
stock, $0.01 par value per share (the "Common Stock"), and (iv) warrants to
purchase Debt Securities, shares of Preferred Stock or shares of Common Stock
(the "Warrants"). The shares of Preferred Stock and Common Stock referred to
above also include such indeterminate number of shares of Preferred Stock or
Common Stock as may be issued upon conversion or exchange of any convertible or
exchangeable Debt Securities or Preferred Stock or upon exercise of the
Warrants, including such shares of Preferred Stock or Common Stock as may be
issued pursuant to anti-dilution adjustments, in amounts, at prices and on terms
to be determined at the time of offering (the"Indeterminate Stock"). The Debt
Securities, the Preferred Stock, the Common Stock, the Warrants and the
Indeterminate Stock are collectively referred to herein as the "Offered
Securities."

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The Board of Directors
Natural MicroSystems Corporation
August 18, 2000
Page 2


         This opinion is being delivered in accordance with the requirements of
Item 601(b)(5) of Regulation S-K under the Act. Capitalized terms used but not
otherwise defined herein have the meanings ascribed to them in the Registration
Statement.

         In connection with this opinion, we have examined originals or copies,
certified or otherwise identified to our satisfaction, of (i) the Registration
Statement; (ii) a copy of the form of Indenture; (iii) the Amended and Restated
Certificate of Incorporation of the Company (the "Certificate of
Incorporation"); (iv) the By-Laws of the Company, as currently in effect (the
"By-Laws"); (v) a specimen certificate evidencing the Common Stock; and (vi)
certain votes adopted to date by the Board of Directors of the Company (the
"Board of Directors") relating to the registration of the Offered Securities. We
have also examined originals or copies, certified or otherwise identified to our
satisfaction, of such other documents, certificates and records as we have
deemed necessary or appropriate as a basis for the opinions set forth herein.

         In our examination, we have assumed the legal capacity of all natural
persons, the genuineness of all signatures, the authenticity of all documents
submitted to us as originals, the conformity to original documents of all
documents submitted to us as certified or photostatic copies and the
authenticity of the originals of such copies. In making our examination of
documents executed or to be executed by parties other than the Company, we have
assumed that such parties had or will have the power, corporate or other, to
enter into and perform all obligations thereunder and have also assumed the due
authorization by all requisite action, corporate or other, and execution and
delivery by such parties of such documents and that such documents constitute
valid and binding obligations of such parties. We have assumed that the
Indenture will be duly authorized, executed and delivered by the applicable
Trustee and that any Debt Securities that may be issued will be manually signed
by duly authorized officers of the Trustee. In addition, we have assumed that
the terms of the Offered Debt Securities (defined below), the Offered Preferred
Stock (defined below) and the Offered Warrants (defined below) will have been
established so as not to violate any applicable law, the Certificate of
Incorporation or By-Laws or result in a default under or breach of any agreement
or instrument binding upon the Company and so as to comply with any requirement
or restriction imposed by any court or governmental body having jurisdiction
over the Company. We have also assumed that (i) any stock certificates
evidencing the Preferred Stock to be issued will be in a form that complies
with, and the terms of such Preferred Stock will be duly established in
accordance with, the Delaware General Corporation Law (the "DGCL"), and (ii) any
stock certificates evidencing any Offered Common Stock (defined below) issued
will conform to the specimen certificate examined by us and will be duly
executed and delivered. We have also assumed that the number of shares of Common
Stock offered pursuant to the Registration Statement does not exceed, at the
time of issuance, the authorized but unissued shares of Common Stock. As to any
facts material to the opinions expressed herein which were not independently
established or verified, we have relied upon oral or written statements and
representations of other officers of the Company and others.

         We have assumed that the execution and delivery by the Company of the
Offered Debt

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The Board of Directors
Natural MicroSystems Corporation
August 18, 2000
Page 3


Securities and the performance by the Company of its obligations thereunder will
not violate, conflict with or constitute a default under (i) any agreement or
instrument to which the Company or its properties is subject (except that we do
not make the assumption set forth in this clause (i) with respect to the
Certificate of Incorporation or By-laws), (ii) any law, rule or regulation to
which the Company is subject (except that we do not make the assumption set
forth in this clause (ii) with respect to those laws, rules and regulations of
the State of Delaware, the Commonwealth of Massachusetts and of the United
States of America, in each case, that, in our experience, are normally
applicable to transactions of the type provided for by the Registration
Statement, but without our having made any special investigation with respect to
any other laws, rules or regulations), (iii) any judicial or regulatory order or
decree of any governmental authority or (iv) any consent, approval, license,
authorization or validation of, or filing, recording or registration with any
governmental authority.

         We express no opinion as to the laws of any jurisdiction other than the
laws of The Commonwealth of Massachusetts, the General Corporation Law of State
of Delaware and the federal securities laws of the United State of America. To
the extent that the laws of any other jurisdiction govern any of the matters as
which we are opining herein, we have assumed that such laws are identical to the
laws of The Commonwealth of Massachusetts.

         Based on and subject to the foregoing and to the other qualifications
and limitations set forth herein, we are of the opinion that:

         1. With respect to any series of Debt Securities (the "Offered Debt
Securities"), when (i) the Registration Statement, as finally amended (including
all necessary post-effective amendments), has become effective under the Act and
the applicable Indenture has been qualified under the Trust Indenture Act of
1939, as amended; (ii) an appropriate prospectus supplement or term sheet with
respect to the Offered Debt Securities has been prepared, delivered and filed in
compliance with the Act and the applicable rules and regulations thereunder;
(iii) if the Offered Debt Securities are to be sold pursuant to a firm
commitment underwritten offering, an underwriting agreement with respect to the
Offered Debt Securities has been duly authorized, executed and delivered by the
Company and the other parties thereto; (iv) the Board of Directors, including
any appropriate committee appointed thereby, and appropriate officers of the
Company have taken all necessary corporate action to approve the issuance and
terms of the Offered Debt Securities and related matters; and (v) the Offered
Debt Securities have been duly executed and authenticated in accordance with the
provisions of the applicable Indenture and duly delivered to the purchasers
thereof upon payment of the agreed-upon consideration therefor, (1) the Offered
Debt Securities, when issued and sold in accordance with the applicable
Indenture and the applicable underwriting agreement, if any, or any other duly
authorized, executed and delivered valid and binding purchase or agency
agreement, will be valid and binding obligations of the Company;

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The Board of Directors
Natural MicroSystems Corporation
August 18, 2000
Page 4


and (2) if the Offered Debt Securities are convertible into Offered Common
Stock, the Offered Common Stock issuable upon conversion of the Offered Debt
Securities will be duly authorized, validly issued, fully paid and
nonassessable, assuming the issuance of the Offered Common Stock upon
conversion of the Offered Debt Securities has been authorized by all
necessary corporate action, that the Offered Debt Securities have been
converted in accordance with their terms and the terms of the applicable
Indenture and that the certificates evidencing such shares of Offered Common
Stock are duly executed and delivered. In rendering the opinion set forth in
clause (2) of this paragraph 1, we have assumed that, at the time of issuance
of any Offered Common Stock upon conversion of the Offered Debt Securities,
the Certificate of Incorporation, the By-Laws and the DGCL shall not have
been amended so as to affect the validity of such issuance.

         2. With respect to the shares of any series of Preferred Stock (the
"Offered Preferred Stock"), when (i) the Registration Statement, as finally
amended (including all necessary post-effective amendments), has become
effective under the Act; (ii) an appropriate prospectus supplement or term sheet
with respect to the shares of the Offered Preferred Stock has been prepared,
delivered and filed in compliance with the Act and the applicable rules and
regulations thereunder; (iii) the terms of the Offered Preferred Stock and of
their issuance and sale have been duly established by all necessary corporate
action in conformity with the Certificate of Incorporation, including a
Certificate of Designation relating to the Offered Preferred Stock, and the
By-Laws; (iv) the filing of a Certificate of Designation with the Secretary of
State of the State of Delaware has duly occurred; (v) if the Offered Preferred
Stock is to be sold pursuant to a firm commitment underwritten offering, an
underwriting agreement with respect to the shares of the Offered Preferred Stock
has been duly authorized, executed and delivered by the Company and the other
parties thereto; and (vi) certificates representing the shares of the Offered
Preferred Stock have been duly executed and delivered by the proper officers of
the Company to the purchasers thereof against payment of the agreed-upon
consideration therefor in the manner contemplated in the Registration Statement
or any prospectus supplement or term sheet relating thereto, (1) the shares of
the Offered Preferred Stock, when issued and sold in accordance with the
applicable underwriting agreement or any other duly authorized, executed and
delivered applicable purchase agreement, will be duly authorized, validly
issued, fully paid and nonassessable, provided that the consideration therefor
is not less than the par value thereof; and (2) if the Offered Preferred Stock
is convertible into Offered Common Stock, the Offered Common Stock issuable upon
conversion of the Offered Preferred Stock will be duly authorized, validly
issued, fully paid and nonassessable, assuming the issuance of the Offered
Common Stock upon conversion of the Offered Preferred Stock has been authorized
by all necessary corporate action, that the Offered Preferred Stock has been
converted in accordance with the terms of a duly authorized, executed and filed
Certificate of Designation and that any certificates evidencing such shares of
Offered Common Stock are duly executed and delivered. In rendering the opinion
set forth in clause (2) of this paragraph 2, we have assumed that, at the time
of issuance of any Offered Common Stock upon conversion of the Offered Preferred
Stock, the Certificate of Incorporation, the By-Laws and the DGCL shall not have
been amended so as to

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The Board of Directors
Natural MicroSystems Corporation
August 18, 2000
Page 5


affect the validity of such issuance.

         3. With respect to the shares of Common Stock (the "Offered Common
Stock"), when (i) the Registration Statement, as finally amended (including all
necessary post-effective amendments), has become effective under the Act; (ii)
an appropriate prospectus supplement or term sheet with respect to the shares of
the Offered Common Stock has been prepared, delivered and filed in compliance
with the Act and the applicable rules and regulations thereunder; (iii) the
terms of the issuance and sale of the Offered Common Stock have been duly
established by all necessary corporate action in conformity with the Certificate
of Incorporation and the By-Laws; (iv) if the Offered Common Stock is to be sold
pursuant to a firm commitment underwritten offering, an underwriting agreement
with respect to the shares of the Offered Common Stock has been duly authorized,
executed and delivered by the Company and the other parties thereto; and (v) any
certificates representing the shares of the Offered Common Stock have been duly
executed and delivered by the proper officers of the Company to the purchasers
thereof against payment of the agreed-upon consideration therefor in the manner
contemplated in the Registration Statement or any prospectus supplement or term
sheet relating thereto, the shares of the Offered Common Stock, when issued and
sold in accordance with the applicable underwriting agreement or any other duly
authorized, executed and delivered applicable purchase agreement, will be duly
authorized, validly issued, fully paid and nonassessable, provided that the
consideration therefor is not less than the par value thereof.

         4. With respect to the Warrants to purchase Debt Securities, shares of
Preferred Stock or shares of Common Stock (the "Offered Warrants"), when (i) the
Registration Statement, as finally amended (including all necessary
post-effective amendments), has become effective under the Act; (ii) an
appropriate prospectus supplement or term sheet with respect to the Offered
Warrants has been prepared, delivered and filed in compliance with the Act and
the applicable rules and regulations thereunder; (iii) the terms of the issuance
and sale of the Offered Warrants have been duly established by all necessary
corporate action in conformity with the Certificate of Incorporation and the
By-Laws; (iv) a debt warrant agreement or stock warrant agreement, as the case
may be, relating to such Warrants in a form to be filed as an exhibit to the
Registration Statement has been duly authorized, executed and delivered by the
Company, the warrant agent appointed by the Company and any other parties
thereto; and (v) such Offered Warrants have been duly executed and delivered by
the proper officers of the Company to the purchasers thereof against payment of
the agreed-upon consideration therefor in the manner contemplated in the
Registration Statement or any prospectus supplement or term sheet relating
thereto, (1) the Offered Warrants, when issued and sold in accordance with the
applicable debt warrant agreement, stock warrant agreement or any other duly
authorized, executed and delivered applicable purchase agreement, will be valid
and binding obligations of the Company; and (2) if the Offered Warrants

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The Board of Directors
Natural MicroSystems Corporation
August 18, 2000
Page 6


are convertible into Offered Common Stock or Offered Preferred Stock, the
Offered Common Stock or Offered Preferred Stock issuable upon conversion of the
Offered Warrants will be duly authorized, validly issued, fully paid and
nonassessable, assuming the issuance of the Offered Common Stock or the Offered
Preferred Stock upon conversion of the Offered Warrants has been authorized by
all necessary corporate action, that the Offered Warrants have been converted in
accordance with their terms and the terms of the applicable warrant purchase
agreement and that the certificates evidencing such shares of Offered Common
Stock or Offered Preferred Stock are duly executed and delivered. In rendering
the opinion set forth in clause (2) of this paragraph 4, we have assumed that,
at the time of issuance of any Offered Common Stock or Offered Preferred Stock
upon conversion of the Offered Warrants, the Certificate of Incorporation, the
By-Laws and the DGCL shall not have been amended so as to affect the validity of
such issuance.

         We hereby consent to the filing of this opinion with the Commission as
an exhibit to the Registration Statement. We also hereby consent to the use of
our name under the heading "Legal Matters" in the prospectus which forms a part
of the Registration Statement. In giving this consent, we do not thereby admit
that we are within the category of persons whose consent is required under
Section 7 of the Act or the rules and regulations of the Commission promulgated
thereunder. This opinion is expressed as of the date hereof, and we disclaim any
undertaking to advise you of any subsequent changes in the facts stated or
assumed herein or of any subsequent changes in applicable law.


                                              Very truly yours,

                                              s/ Choate, Hall & Stewart