Priceline.com Inc.'s (PCLN) growth slowed markedly in the second quarter, with a surprisingly downbeat outlook underscoring how its robust gains in the European hotel market are falling prey to economic uncertainty.

As the online travel agency reported a 37% earnings increase, its stock went into a tailspin after hours on the unwinding growth and cautious outlook. Shares were down 15% at $580.50 and dragged down others in the sector. Expedia Inc. (EXPE) traded down 3.8% to $56.65, TripAdvisor Inc. (TRIP) fell 4.6% to $36.90, and Orbitz Worldwide Inc. (OWW) declined 2.8% to $4.53.

Among online travel agencies, Priceline has the greatest international and hotel focus, which until now have been the sector's breeding grounds for growth.

Priceline Chief Executive Jeffery H. Boyd said the company--operator of its namesake website as well as Booking.com, Agoda.com and Rentalcars.com--now assumes fairly significant deceleration, particularly in the key European businesses that represent about 60% of its booked hotel room nights.