Amazon has reported a loss of $126m (£74m) in the second quarter and warned that sales could slow in the current quarter.
Amazon forecast third quarter sales of between $19.7bn and $21.5bn, which could mean sales growth of as little as 15% - well down on previous quarters.
Amazon has traditionally survived on thin profit margins, but investors have been reassured by strong sales growth.
But today's warning over sales has spooked investors.
In after hours trading in the US shares slumped by 6%.