Apple Inc., SanDisk Corp. and Samsung Electronics Co. are being sued by a closely held Texas company that claims they are infringing its patent for portable music players.

Texas MP3 Technologies Ltd., which shares a street address with one of its lawyers in Marshall, Texas, filed the lawsuit Feb. 16 in federal court there. Its patent, issued in June, claims to cover an "MPEG portable sound reproducing system."

The suit targets the music players made by the three companies, including Apple's top-selling iPod and SanDisk's Sansa. The U.S. market for portable music players was $6.1 billion last year, with about 37 million units sold, according to market researcher NPD Group.