iTunes has sold four billion songs since its 2003 launch
Apple's iTunes has overtaken supermarket group Wal-Mart to become the largest music retailer in the US, an independent study has said.
Market research firm NPD said iTunes surpassed Wal-Mart in January and February if 12 downloads are considered equal to the sale of one CD album.
Steve Knopper, a reporter for Rolling Stone magazine, said the news showed the big change in music retailing.
iTunes has sold more than four billion songs since its launch in 2003.
Eddy Cue, iTunes vice president, said the Apple unit was "thrilled" by the findings of the NPD survey.
More than 50 million customers have now used iTunes since it started.
While iTunes continues to dominate the music download market, it faces a growing number of competitors.
Social networking site MySpace is just the latest to join the market, launching an online music service in partnership with three major music companies - Universal, Sony BMG and Warner.
ITunes' more established rivals include eMusic and Napster.
MySpace users will now be able to listen to music and watch music videos free of charge, only paying for what they want to download.
Record business analyst Michael Gartenberg of Jupiter Research said iTunes surpassing Wal-Mart "underscores a very important trend in the industry".
"It wasn't that long ago that Apple was proud of the fact that they were number four and rising and then number three and number two."
More than 50 million customers have used iTunes since it started.