American Apparel Inc. said on Thursday that same-store sales grew 24 percent in the third quarter, including a 15 percent increase in the back-to-school month of September.

The Los Angeles-based clothing company, which opened 33 new stores in the third quarter, based its figures on 150 locations that had been open for more than 12 months, considered a more reliable metric of retail health.

The company said it plans to file its quarterly earnings report with the Securities and Exchange Commission on Nov. 10, but has not set a date for a conference call.

American Apparel shares closed down 31 cents, or 4.8 percent, to $6.10 on the American Stock Exchange.