Natural MicroSystems Corporation

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20459

FORM 10-K

(Mark One)
[X] ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE
ACT OF 1934 [NO FEE REQUIRED] For the fiscal year ended December 31, 1999

[ ] TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1994 [NO FEE REQUIRED] For the transition period from _________ to _________

Commission File No. 0-23282

NATURAL MICROSYSTEMS CORPORATION
(Exact name of registrant as specified in its charter)

Delaware                                                                             04-2814586
(State or other jurisdiction of incorporation or organization)        (I. R. S. employer identification number)

100 Crossing Boulevard, Framingham, Massachusetts        01702
(Address of principal executive office)                                     (Zip code)

Registrant’s telephone number, including area code: 508-620-9300

Securities registered pursuant to Section 12(b) of the Act: None

Securities registered pursuant to Section 12(g) of the Act:

Title of class                                                             Name of each exchange on which registered Common Stock, $.01 per share                                NASDAQ National Market