Shares of warehouse club operator Costco Wholesale Corp. (NASDAQ: COST) are lower this morning, despite posting an increase of 31% in its fiscal second-quarter profit.

For the quarter, the largest U.S. wholesale club reported that its profit surged to $327.9 million, compared with $249.5 million in the same period a year ago. Last-year earnings included $53.4 million related to non-recurring charges. Higher gasoline prices boosted the company's profit in the quarter and Costco posted earnings of 74 cents per share, which was in line with analysts' forecasts.

Costco also announced a respectable 12% increase in revenues, to $16.62 billion, up from $14.80 billion a year earlier. Analysts were expecting the company to show $16.85 billion in revenue, according to Thomson Financial.

Looking at same stores sales (sales at stores open at least 12 months), the quarter saw growth of 7%. Increased demand for gasoline resulted in gains of 5% in the U.S. same-store sales and 17% in international sales, which were boosted by surging gas prices and a weaker U.S. dollar.

Despite a difficult U.S. retail sector, Costco gained market share. Fighting against a weak U.S. economy and continued fears over a possible recession, consumers searched for a relief in lower food and gas prices at the warehouse store.

Even so, the company's numbers did not impress traders, who were disappointed by Costco's lower-than-expected revenue. They pushed the stock down over 2% in morning trading, although shares are fighting their way back up as the morning progresses.

Eliza Popescu is a financial writer for the online investment advisory service Investor's Observer.