Shares in Internet search giant Google fell as much as 10% on Tuesday after finance officer George Reyes said overall growth was slowing.
He told investors at a conference that the company would have to find new ways to grow revenues.
Nearly all Google advertising revenue comes from a pay-per-click ads system.
Shares in Google were down more than $39 to about $350 on Nasdaq at one stage before closing down 7.11%, or $27.76, at $363.40.
"Growth is slowing and now largely organic," Mr Reyes was quoted as saying.
"The search monetisation gains have now been largely realised."
Tim Biggam, chief options strategist at Man Securities in Chicago, said that stock valuations based on future growth prospects were also likely to be hit when expectations were dampened.
"As in the case with Google... you are going to see this sort of negative reaction in the marketplace."
Google stock had been hit in January over fears that its shares were overvalued. Then, its 82% surge in profits failed to meet Wall Street's forecasts and questions were asked about its way forward.
At the time, it reported that net income in the last three months of 2005 rose to $372.2m (£209m) from $204.10m a year earlier, lifted by a strong demand for online advertising.
"Google is taking a shaky market down. It's the other shoe to drop," said another analyst, Larry Peruzzi, senior equity trader at The Boston Company Asset Management.
"It's been a stellar performer and a leader," he added. "This is kind of new territory for them being this cautious."