Apple tied for the second spot with LimeWire, a P2P service, The NPD Group said. The No. 1 service in March was WinMX, a P2P network that was used by 2.1 million households to download music. Apple and LimeWire were used by 1.7 million households.
Napster and Real Networks, which offer subscription services, as well as music downloads, also placed in the top 10 sites.
"These digital download stores appear to have created a compelling and economically viable alternative to illegal file sharing," Russ Crupnick, president of NPD's music and movies division, said in a statement.
In total, 4 percent of Internet-enabled U.S. households used an online music store in March. A large number of the consumers who paid for music were 30 years old or older. The average age was 33, and average household income was $83,000, NPD said.
The study showed that illegal music downloads decreased steadily after people left college, a time when they have less free time and more money to spend.
"Those that had tried digital music through file sharing were slowing down or stopping that illegal behavior, and many post-college consumers are leading the charge into legal a la carte downloading," Crupnick said.
The top ten digital music services, based on the number of households that acquired a digital song in March, were, in order, WinMx, ITunes, LimeWire, Kazaa, BearShare, Ares Galaxy, Napster, Morpheus, Real Player Store and IMesh.