The lawsuit caps a bad week for the small company, which lost the rights to its Aimster.com domain name to AOL Time Warner in an arbitration panel Monday.
The Recording Industry Association of America, which has previously sued file-trading companies Napster and Scour on similar issues, sent a warning letter to Aimster several months ago, noting that the company's activities were similar enough to Napster's that it was likely in violation of the law. But unlike other file-swapping companies that received similar letters, Aimster has declined to filter songs or monitor its members' activities.
The Motion Picture Association of America was expected to file its own separate suit later.
"Aimster is Napster all over again," Cary Sherman, general counsel for the RIAA, said in a statement. "Beneath the added bells and whistles lies the same service that Napster provides."
In an unrelated action, record companies sued online radio company Launch Media, alleging it was violating copyright laws by allowing Webcast listeners too much control over what and when they wanted to hear specific songs or bands.
Like Napster or any of its other rivals, Aimster allows individuals with computers connected to the Internet to trade files that are stored on their hard drives.
The company started with a different twist on the idea of file sharing, allowing people to create "buddy lists" such as those on AOL Instant Messenger and to open their computers just to this select group of people. By creating these small, private networks, Aimster said it was preserving people from outside scrutiny.
But for the last month or two, the company has also offered a more sprawling, Napster-like service, in which people can search for music, video or software across a network comprising tens of thousands of people. New Aimster users were automatically added into this network unless they configured their software specifically for the smaller buddy list trading.
Aimster CEO Johnny Deep has been among the most vocal bomb-throwers in the file-trading community over the last few months. When Napster began using filters, Aimster released a software program that turned file names into Pig Latin, serving as a temporary way around the blocking mechanism. It later took this down at Napster's request.
More recently, the company went to court, asking for a judgment that its "private virtual networks" were legal. Unlike Napster, the company and the people who use its software do more than trading music files, and breaking into these personal private networks to look for copyrighted files would itself be a violation of copyright law, company attorneys contended. A judge has yet to rule on that motion.
Deep is personally named in the suit as a representative of the Aimster service--as are BuddyUSA and AbovePeer, corporate entities that own the Aimster software and file-swapping service, respectively.
Deep declined to comment specifically on the suit, saying the company hadn't yet seen it. But "we already have a motion that asks a federal court for guidance in deciding how to balance rights to privacy and free speech with protection of copyright," he said.
In an interview with CNET News.com earlier this week, Deep said that he personally was willing to fight a full RIAA lawsuit but that such action was "a question for everybody involved" with the company.