During the past three months, iTunes made it into the US top 10 sales list for the first time, NPD said.
Apple claims to have sold more than 600 million songs to its 10 million subscribers, and has sold 30 million iPods since launching the device in 2001.
According to figures from the Recording Industry Association of America, digital sales accounted for four per cent of the market during the first half of 2005, up from about 1.5 per cent during the first half of 2004.
In other news, domain name registrar Nominet has announced that Ben Cohen, former teenaged dotcom millionaire, has relinquished his fight to regain the itunes.co.uk domain.
Nominet company solicitor Edward Phillips said that Cohen's company, CyberBritain Group, has abandoned all further attempts to get the domain it once owned back from Apple.
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