Shares of Crocs skidded as much as 51 percent in after-hours U.S. trading Thursday after the maker of colorful clogs sharply lowered second-quarter guidance.

Shares of Niwot-based Crocs plunged as much as $4.50 to $4.40 in after-hours trading on the Nasdaq Stock Market. That's less than half Crocs' split-adjusted February 2006 IPO price of $10.50.

The sell-off came after Crocs lowered its earnings guidance for the second quarter, saying the U.S. market turned out to be "more challenging" than originally anticipated. Crocs now expects second-quarter revenue to be in the range of $218 million to $223 million and diluted earnings per share in the range of 3 cents to 7 cents. That's down from the earlier guidance of revenues in the range of $247 million to $258 million, with earnings per share of 42 cents to 47 cents.

"We are obviously disappointed with the economic situation in the U.S. and parts of Europe. However, we remain confident about the long-term prospects of this business," said Crocs CEO Ron Snyder in a statement. "We are currently in the process of sizing our business to be profitable on lower projected sales volumes and these cost actions will continue through the end of the year."

For the full year, Crocs said revenues "are now expected to be down modestly compared to 2007" with diluted earnings per share about break-even.