Trading in the Nasdaq, a major stock exchange dominated by the biggest names in technology, was halted shortly after midday Thursday due to a technical glitch. Full trading was set to resume later Thursday.
Other exchanges were operating normally, and stocks rose slightly.
The disruption sent brokers scurrying to figure out what went wrong and raised new questions about the pitfalls of computer-driven stock trading.
Thursday's Nasdaq freeze echoed earlier stock market snafus, such the sudden plunge in stocks in May 2010 that came to be known as the "flash crash" and the glitch-plagued initial public offering of Facebook last year.
The exchange sent out an alert to traders shortly after noon EDT saying that trading was being halted until further notice because of problems with a quote dissemination system. Later, Nasdaq said it would resume trading in phases in the afternoon, with full trading expected to restart around 3:25 p.m. EDT.
Nasdaq said it wouldn't be canceling any open orders, but that customers could cancel orders if they wanted to.
Securities and Exchange Commission spokesman John Nester said: "We are monitoring the situation and are in close contact with the exchanges."
The Nasdaq composite index was stuck at 3,631.17, up 31.38 for the day. The index is up 20 percent so far this year.