The deal combines Jupiter, a leading Internet market-research firm, with Media Metrix, the most closely followed provider of Web-site usage figures.

The merged company will be called Jupiter Media Metrix.

Under the terms of the deal, Jupiter shareholders will receive 0.946 Media Metrix shares for each Jupiter share they own. Shares of Jupiter closed Monday at $24, down $1.1875, while Media Metrix closed at $28.25, down 87.5 cents. Both trade on the Nasdaq Stock Market.

As of April 30, there were 15,455,657 shares of the Jupiter's common stock outstanding, according to its most recent quarterly report. Using those figures, the acquisition has a value of about $413 million.

The companies said Tod Johnson, chairman and chief executive officer of Media Metrix, will become chairman and chief executive officer of the new company. Gene DeRose, chairman and chief executive officer of Jupiter, will become president and vice chairman of Jupiter Media Metrix.

Mary Ann Packo, president and chief operating officer of Media Metrix, and Kurt Abrahamson, president and chief operating officer of Jupiter, will become co-chief operating officers of Jupiter Media Metrix, and will continue as presidents of their respective business units, the companies said. Tom Lynch, chief financial officer of Media Metrix, will become the new company's chief financial officer, while Jean Robinson, Jupiter's chief financial officer, will become executive vice president, business development.

The transaction is expected to add to earnings at closing on a cash earnings-per-share basis. The boards of both companies approved the merger agreement. Media Metrix was advised by Thomas Weisel Partners on the transaction; Jupiter was advised by Morgan Stanley Dean Witter.