The New York Stock Exchange stopped trading on Wednesday morning because of a technical issue, though shares continued to trade on other exchanges.
The Exchange said in a statement that it was experiencing a "technical issue" and that it was working to resolve it as quickly as possible.
Trading continued on Nasdaq and other exchanges.
US indexes sank amid growing concerns that trouble in China's markets could spread.
"My gut is that there are bigger issues to worry about than a trading glitch," said Associated Press Business Writer Bernard Condon.
He said that the Chinese stock market and the Greece bail out deal were more worrying to investors.