Late Thursday, Gap ( GPS) reported August comparable sales of 2%, slightly below analysts' expectations of 2.2% and below the 9% same-store sales figure reported in the year-earlier quarter. Of the three brands owned by Gap that report monthly comparable sales, both the Gap flagship brand and Banana Republic saw comps rise 2% last month. Analysts' were expecting gains of 2.8% in Gap and a 0.4% decline at Banana. Old Navy's slim 1% comp sales growth fell short of analysts' expectations for a 1.3% gain. Gap shares were trading lower in after-hours, losing 0.98% to $40.25. Shares of Zumiez ( ZUMZ) tumbled 5.6% to $23.86 in post-markets trading following the teen retailer's cautious outlook for the third quarter. Zumiez said its net income more than doubled to $4.7 million, or 16 cents a share, in the second quarter. Net sales for the entire quarter jumped 17% to $157.9 million, however second quarter comparable sales rose just 0.9% compared to 9.5% last year. Zumiez saw a 3% rise in August comp sales also surpassed expectations for 2.5% growth. However, the sport apparel retailer's guidance for the third quarter wasn't reassuring to investors. Zumiez expects net sales in the range of $187 million to $191 million and earnings of 39 cents to 43 cents a share, based on expected comp sales that are flat to up 2%, it said. The average analyst estimate was expecting third-quarter earnings of 55 cents a share, according to Thomson Reuters. Earlier in the day, Costco Wholesale ( COST), the biggest company that still reports monthly sales, said August sales came in above the average analyst estimate. Shares closed up 2.8% to $114.62 on Thursday. The Issaquah, Wash.-based company reported net sales of $7.95 billion for the four weeks ended Sept. 1, up 7% from August 2012. Comparable sales for the four-week period rose 4% vs. 3.8% expected by analysts. Excluding changes in gas prices and foreign exchange rates, the company posted comparable sales of 6% for the month.