Natural MicroSystems Corporation

Indicate by check mark whether the registrant:
(1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such report), and
(2) has been subject to such filing requirements for the past 90 days. YES X NO ___

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained to the best of the registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K. [ ]

As of January 21, 2000, the aggregate market value of the voting stock held by non-affiliates of the registrant was approximately $547.5 million, based on the closing price on such date of the registrant’s Common Stock on the NASDAQ National Market. As of January 21, 2000, 12,777,327 shares of Common Stock, $.01 par value per share were outstanding.

Documents Incorporated By Reference
Portions of the registrant’s Proxy Statement relating to the 2000 Annual Meeting of Stockholders of the registrant are incorporated into Part III of this Form 10-K. The exhibits listed in the accompanying Exhibit Index beginning on page 48 are filed or incorporated by reference as part of this Form 10-K.

EXHIBIT INDEX ON PAGE 48

The following are trademarks and trade names of the company indicated.
CT Access, Fusion, Natural Access, Natural Call Control, Convergence Generation, NaturalEdge, NaturalFax, Open Telecommunications, QWES, QWES.com and PolicyPoint are trademarks; Alliance Generation is a registered trademark; and Natural Microsystems is a trade name of the registrant. All other brand names or trademarks appearing in this Form 10-K are the property of their respective holders.

This Form 10-K, future filings of the registrant, press releases of the registrant and oral statements made with the approval of an authorized executive officer of the registrant may contain forward-looking statements. In connection therewith, please see the cautionary statements and risk factors contained in Item 1, “Business - Forward-Looking Information” and “Business - Risk Factors,” which identify important factors which could cause actual results to differ materially from those in any such forward-looking statements.

References in this Form 10-K to the “Company,” the “registrant,” “we,” “our” and “us” refer to Natural Microsystems Corporation and its subsidiaries.