SAN FRANCISCO — Google Inc.'s shares plummeted by as much as 13 percent yesterday after the company's chief financial officer raised the specter of slower earnings growth — a remark that soured recent investor enthusiasm that had been building for the Internet's leading search engine.

The latest in a series of abrupt downturns in Google's stock followed CFO George Reyes' answer to a question during an investor conference hosted by Merrill Lynch in New York.

After hailing the results of an 18-month effort to boost advertising revenue, Reyes predicted it will become increasingly difficult for Mountain View, Calif.-based Google to maintain its rapid growth pace.

"Most of what's left is just organic growth, which means you have to find ways to grow your traffic," Reyes said.

Google's shares closed down $27.76, or 7.1 percent, at $362.62. The stock, which went public 18 months ago at $85 per share, peaked at $475.11 in early January.