Gap (GPS) said a key sales measurement fell in August, and the retailer's shares slid in aftermarket trading.

The San Francisco company said sales at stores open at least one year fell 2 percent over the four weeks that ended Aug. 30. Gap Inc. said revenue from Gap stores open over that time fell 6 percent, which will pressure the brand's profit margins in September.

The revenue metric slipped 2 percent for Banana Republic and rose 2 percent at Old Navy in August.

Sales at locations open at least a year is considered an important measurement of retailer health because it strips out results from stores that opened or closed in the last year, which can skew trends.

Gap said its total sales were unchanged at $1.23 billion.

Its shares skidded $2.79, or 6 percent, to $43.80 in aftermarket trading.