May was very kind to Motley Fool Stock Advisor selection Costco (NASDAQ:COST) wherever its wholesale stores were open. Its same-store sales were up 7%, with both domestic and international stores reporting the same 7% increase.

The 7% increase in same-store sales surpassed the 4.1% gain posted by BJ's Wholesale Club (NYSE:BJ) and the 5.4% gain reported by Wal-Mart (NYSE:WMT) unit Sam's Club. These results pretty much sum up how the industry has played out for years. Costco leads the way, Sam's Club is a credible No. 2, and BJ's, while still a good business with some regional strength, lags the pack.

For Costco, the strongest regions in the U.S. were the Midwest, Texas, the Northeast, and its home, the Northwest. Internationally, Korea and Taiwan were the country's strongest markets, based on local currency. Overall same-store sales received a 0.35% boost from currency gains.

The number of selling days in May this year and last year were the same, so on a year-over-year basis, there are no calendar irregularities driving this year's results. If you're looking ahead to June, there is good reason to be encouraged with the sales strength at Costco heading into the July Fourth holiday. Considering the overall trends in the market lately and some soft results among retailers, it's nice to see Costco keep chugging along.

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Nathan Parmelee owns shares of Costco, but had no financial interest in any of the other companies mentioned. Wal-Mart is an Inside Value selection. The Motley Fool has an ironclad disclosure policy.