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.
