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 Thursday.

Best Buy Co. was ranked behind Wal-Mart and iTunes, with Amazon.com and Target tied for fourth 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.

Apple's rise in the NPD survey also reflects a trend this decade of declining CD sales and rising digital music sales, which favors digital music retailers.