LOS ANGELES — Apple Inc.'s iTunes online music store vaulted past Wal-Mart Stores Inc. in February to become the top overall music retailer in the U.S., a market research firm said yesterday.

Best Buy Co. was ranked behind iTunes and Wal-Mart, with www.Amazon.com and Target tied for the fourth spot in January and February, according to consumer surveys conducted by The NPD Group.

The firm tabulated units sold, counting every 12 digital downloads as one CD. It did not count sales revenue, nor mobile music sales.

Cupertino, Calif.-based Apple touted the latest signs of its music retail dominance, noting it has more than 50 million customers. A survey NPD released in February covering annual sales proclaimed iTunes leapfrogged to the No. 2 ranking in 2007.

Chief Executive Officer Steve Jobs opened the iTunes store in April 2003 to help spur iPod sales, and has sold more than 4 billion tracks since its launch.

The site now carries a catalog of more than 6 million downloadable tracks, up from 200,000 five years ago.

Amazon.com, the world's biggest Internet retailer, started its music service in September. The Seattle-based company now offers tracks without copy protection from the world's four largest labels: Vivendi SA's Universal Music Group, Sony BMG Music Entertainment, Warner Music Group Corp. and EMI Group. Apple sells songs without protection from only one label, EMI.