Google's profits rose by less than expected between April and June, raising concerns of a slowdown in the online advertising market.
The internet giant posted a 35% rise in second quarter net profit to $1.25bn (£626m) - slightly below forecasts.
The company said its business had held up well despite a more challenging economic environment.
But investors, used to Google outperforming, sent the firm's shares 8% lower in US after-hours trading.
"There's the initial shock of this being the best company in the space and it just fell short, said Colin Gillis, analyst at Canaccord Adams.
Google reported a 39% rise in revenue to $5.37bn in the three months to 30 June.
"Strong international growth as well as sustained traffic increases on Google's web properties propelled us to another strong quarter, despite a more challenging economic environment," said Eric Schmidt, Google's chief executive.
The company blamed its below-forecast performance on lower returns from investing its $12.7bn cash pile as a result of volatile interest rates.
The number of paid clicks on websites operated by Google and its partners fell 1% from the first quarter.
Stanford Group analyst Clayton Moran said it was evidence "there is a slowdown in internet advertising that's affecting Google".
Google employed another 448 people during the quarter - the fewest hired since the fourth quarter of 2004.