BlackBerry maker Research In Motion Ltd. warned investors to expect quarterly profit at the low end of its forecast, despite outsized subscriber growth, providing more evidence that RIM's drive to gain market share is eating away at its profit margins.

Investors reacted by dumping shares of the Waterloo, Ontario-based company. The stock lost 15%, or $8.28, to $48.76 at 4 p.m. Wednesday on the Nasdaq Stock Market.

RIM...