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SCHEDULE 13G
(RULE 13d-102)
Information to be Included in Statements Filed Pursuant to Rule 13d-1(b), (c)
and (d) and Amendments Thereto Filed Pursuant to Rule 13d-2.
SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, DC 20549
SCHEDULE 13G
Under the Securities Exchange Act of 1934
(Amendment No. ___________)*
Natural Microsystems Corp.
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(Name of Issuer)
Common
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(Title of Class of Securities)
638882100
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(CUSIP Number)
December 31, 1999
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(Date of Event Which Requires Filing of this Statement)
Check the appropriate box to designate the rule pursuant to which this
Schedule is filed:
[ ] Rule 13d-1(b)
[ ] Rule 13d-1(c)
[X] Rule 13d-1(d)
*The remainder of this cover page shall be filled out for a reporting
person's initial filing on this form with respect to the subject class of
securities, and for any subsequent amendment containing information which would
alter the disclosures provided in a prior cover page.
The information required in the remainder of this cover page shall not
be deemed to be "filed" for the purpose of Section 18 of the Securities Exchange
Act of 1934 ("Act") or otherwise subject to the liabilities of that section of
the Act, but shall be subject to all other provisions of the Act (however, see
the Notes).
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CUSIP NO. 13G PAGE ___ OF ___ PAGES
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1 Names of Reporting Persons/I.R.S. Identification Nos. of Above Persons
(Entities Only)
Driehaus Capital Management, Inc. 36-3196026
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2 Check the Appropriate Box if a Member of a Group (a) [ ]
(See Instructions) (b) [ ]
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3 SEC Use Only
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4 Citizenship or Place of Organization
Illinois, U.S.
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Number of
5 Sole Voting Power
Shares 340,884
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Beneficially 6 Shared Voting Power
0
Owned by -----------------------------------------------------
7 Sole Dispositive Power
Each 706,403
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Reporting 8 Shared Dispositive Power
Person With 0
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9 Aggregate Amount Beneficially Owned by Each Reporting Person
706,403
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10 Check if the Aggregate Amount in Row (9) Excludes Certain Shares [ ]
(See Instructions)
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11 Percent of Class Represented by Amount in Row (9)
6.31
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12 Type of Reporting Person (See Instructions)
IA
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Driehaus Capital Management, Inc.
SCHEDULE 13G
Item 1. (a) Name of Issuer: Natural Microsystems Corp
(b) Issuer's Principal Executive Offices:
100 Crossing Blvd.
Framingham, MA 01702
Item 2. (a) Person Filing - Driehaus Capital Management, Inc.
(b) Address - 25 East Erie Street
Chicago, IL 60611
(c) Citizenship - U.S.A.
(d) Title of Class of Securities - Common
(e) CUSIP Number - 638882100
Item 3. (e) Investment Adviser Registered under section 203 of
the Investment Advisers Act of 1940.
Item 4. (a) Amount Beneficially Owned - 706,403
(b) Percent of Class - 6.31%
(c) Number of shares as to which reporting person has:
(i) Sole voting power 340,884
(ii) Shared voting power 0
(iii) Sole dispositive power 706,403
(iv) Shared dispositive power 0
Item 5. Not Applicable
Item 6. The reporting person, Driehaus Capital Management,
Inc., manages accounts for numerous clients on a fully
discretionary basis. Securities held in these clients'
accounts are included in this report to reflect the
fact that the reporting person may have voting power or
dispositive power. To the best of the reporting
person's knowledge, none of the clients whose
securities are included in this report have an
ownership interest in the subject class of securities
which exceeds five percent of the class.
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Driehaus Capital Management, Inc.
SCHEDULE 13G
Item 7. Not Applicable
Item 8. Not Applicable
Item 9. Not Applicable
Item 10. Certification
By signing below I certify that, to the best of my knowledge and
belief, the securities referred to above were acquired and are held in
the ordinary course of business and were not acquired and are not held
for the purpose of or with the effect of changing or influencing the
control of the issuer of the securities and were not acquired and are
not held in connection with or as a participant in any transaction
having that purpose or effect.
SIGNATURE
After reasonable inquiry and to the best of my knowledge and belief,
I certify that the information set forth in this statement is true,
complete and correct.
February 11, 2000
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Date
/s/ Mary H. Weiss
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Signature
Mary H. Weiss, Vice President
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Name/Title
