Jeff Greenberg, chairman of the consulting firm MMC, said more than 1,000 people in its 1,700-person work force in the twin towers were safe, but others are still being sought.

''We are in touch with all the local hospitals and expect information from them,'' he said in a message on the company's Web site. ''As we learn more about the status and whereabouts of our colleagues, we will let you know.''

Morgan Stanley Dean Witter & Co. chairman and chief executive Philip Purcell said a ''majority'' of his company's 3,700 workers in the trade center got out alive.

''We have some missing people that we are looking for,'' he said.

The company occupied 22 floors in the south tower.

In a statement on the company's Web site, Purcell said the company will ''resume full operations as exchanges and markets reopen.''

The twin towers were home to such symbols of Western economic might as the Bank of America, Kemper Insurance, Lehman Brothers, Morgan Stanley Dean Witter, Credit Suisse First Boston and Sun Microsystems.

Normally 50,000 people work in the twin towers, but the first attack came when many workers were not yet in their offices. Officials estimated that 10,000 to 20,000 people were in the buildings when the first plane crashed.

Jim Connelly, a spokesman for Fred Alger Management Inc., said 38 people in its 235-member global work force were on the 93rd floor during the attacks. They haven't been found.

Among the missing is David Alger, the company's president and the brother of founder Fred Alger.

''The terrorist attack is a personal tragedy for my family as well as for all of our employees and their families,'' said Fred Alger, who has assumed leadership of the company.

Sun Microsystems, which leased two floors, said its 340 workers survived the attack. But Phil Rosenzweig, a director in its software organization was among those killed on American Airlines Flight 11, which crashed into the World Trade Center.

''Although we are relieved that our World Trade Center employees are safe, we are devastated by the tremendous losses that are being suffered,'' said Scott McNealy, Sun's chairman and chief executive.

The investment firm Lehman Brothers said it evacuated nearly 6,000 of its workers after the first plane struck the center. It had two floors in the north tower.

Chicago-based insurer Aon Corp. said 1,100 of its 50,000 workers had offices in the south tower.

Aon's chairman and chief executive, Patrick G. Ryan said the company is focusing on its ''employees and their families.''

Like other companies, Ryan said the company's New York offices will be closed until further notice.

Merrill Lynch said it evacuated its 9,000 workers from offices in the Financial Center and other offices in lower Manhattan, the company said in a press release.

Dozens of foreign companies had offices in the towers, including Sinochem American Holdings of China; Japan's Nikko Securities; Zim-American Israeli Shipping Co. of Israel and Cantor Fitzgerald International of London.

Cantor Fitzgerald, and eSpeed International, the electronic trading service which was spun off by Cantor, said they were still taking stock of the tragedy. Both companies had operations on the 101st and 103rd to 105th floors, and employed 1,000 workers there.

''All of our thoughts and prayers are with our New York colleagues and their families and friends at this time,'' said Howard W. Lutnick, chairman of both companies. ''In a very difficult and confused situation we are doing all that we can to determine more about the situations of colleagues.''

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