(Adds company comment on results, rebate checks)
LOS ANGELES, June 5 (Reuters) - Costco Wholesale Corp (COST.O) on Thursday reported a better-than-expected 9 percent rise in May sales at stores open at least a year, helped by high gasoline and food prices as well as the weak dollar.
This was the company's biggest overall same-store sales increase since November, and shares of the largest U.S. warehouse club operator rose more than 3 percent.
Analysts on average had expected a same-store sales gain of 6.9 percent, according to Thomson Reuters data.
Stifel Nicolaus analyst David Schick said Costco, with its focus on selling value-priced goods, was a relatively safe haven for investors.
Customers pay an annual fee to shop at the company's warehouse locations, which sell everything from digital cameras and watches to bulk-sized packages of toilet paper and soda.
The core Costco business, which excludes gas and foreign exchange benefits, remains robust, Schick said.
The company did say declining tobacco sales and competition among stores in the chain dragged on results.
Net sales for the four weeks ended on June 1 rose 12 percent to $5.77 billion.
Costco and discount retailer Wal-Mart Stores Inc (WMT.N) have outperformed the retail sector as consumers cut back in response to a weak U.S. economy, tight credit and soaring prices for necessities like food and gasoline.
May sales at Wal-Mart also topped analysts' targets, and the world's largest retailer on Thursday said shoppers cashed about $350 million worth of tax rebate checks in its stores during the month.
Costco, which targets more upscale shoppers than Wal-Mart, did not cash rebate checks.
Still, both companies said the checks appeared to have a positive effect on sales.
"There was probably some benefit in there, but measuring it was impossible," Costco spokesman Bob Nelson said.
Same-store sales increased 7 percent at Costco's U.S. locations.
About 2 percentage points of that gain came from higher gas prices, which rose to an average of $3.67 a gallon from $3.09 in May 2007. With prices now above $4 per gallon, the company expects an even bigger boost to sales in June.
Costco's strongest domestic results came out of the Southeast, Midwest, Northwest and Texas regions, the company said in a recorded conference call.
It said it saw a "continued slight improvement" in California, although the nation's most populous state continued to lag the rest of the United States in terms of comparable-sales performance.
At international locations, same-store sales jumped 15 percent, with the biggest positive impact coming from foreign exchange rates, primarily in Canada. On a local currency basis, international same-store sales increased 8 percent.
Costco's food business remained strong, with same-store sales rising at a high single-digit percentage rate.
The company said sales were down somewhat in housewares, home furnishings and apparel. Tobacco and cannibalization each reduced overall comparable sales by 0.8 percentage points.
Traffic was up a little more than 3 percent in May.
The company has 537 stores worldwide. By the end of its fiscal year on Aug. 31, it plans to open an additional seven or eight, including the relocation of two stores.
Costco shares were up $2.45, or 3.5 percent, at $73.26 in afternoon Nasdaq trade. On the New York Stock Exchange, Wal-Mart rose 3.4 percent to $59.65. (Reporting by Lisa Baertlein and Tenzin Pema; Editing by David Holmes and Lisa Von Ahn)