Yahoo shares have fallen nearly 11% after the US search firm said weakness in financial and car advertising could hit its third quarter sales.
Yahoo has already warned of lower growth this year
A senior Yahoo executive said quarterly revenues would be at the low end of previously stated expectations.
Troubled carmakers Ford and GM have been cutting their advertising budgets as they try to slash overall costs.
Shares in Google fell 4% as investors worried that other online media firms dependent on advertising could be hit.
Susan Decker, Yahoo's chief financial officer, told a conference in New York that bookings from financial services and car advertisers had been weaker in recent weeks.
"It is a new trend," she said.
"We don't know yet if it is an indicator of a broader slowdown. We are seeing it enough to say something but I don't want to overplay it either."
In July, Yahoo forecast third quarter sales in the range of $1.12bn to $1.23bn.
One analyst said reduced expenditure by car companies would affect all media businesses not just internet companies.
"It would be naive to say that advertisers would continue to pour ahead on online advertising and cut back only on traditional advertising in the face of economic weakness," said Martin Pyykkonen, an analyst with Global Crown Capital.
Yahoo shares closed down 10.9% at $25.82.