Christopher J. Steskal, a lead prosecutor of a federal task force investigating the backdating of stock options at Apple Inc. and other companies, is leaving his San Francisco post to join a law firm, the United State Attorney’s office in San Francisco confirmed late Friday.

Mr. Steskal’s decision follows the recent resignation of his boss, United States Attorney Kevin V. Ryan.

Mr. Steskal also was the lead prosecutor in the government’s case against Gregory L. Reyes, the former chief executive of Brocade Communications Systems.

Mr. Steskal, who was to try Mr. Reyes’s case in June in San Francisco, is the second of five assistant United States attorneys on the task force to leave since its formation in July. He said he would join the San Francisco office of Fenwick & West within 30 days.

William J. Portanova, a criminal defense attorney and former federal prosecutor, said that he believed that the Brocade case would not suffer, no matter who is picked to succeed Mr. Steskal.

“The government has people stacked up ready to pick up the case and run with it,” he said.

Luke Macaulay, a spokesman for the United States Attorney’s office in San Francisco, said that one of two prosecutors the office is planning to hire would replace Mr. Steskal on the task force.