MediaBay contends that Napster was guilty of "vicarious copyright infringement and unfair competition" by trading MediaBay radio programs without its permission. The company is seeking an undisclosed amount in damages, and requesting that Napster filter out its programs and block them from being traded.
A MediaBay spokesperson said: "The injunction against Napster is a key step toward levelling the playing field for legitimate downloadable audio companies."
Napster representatives declined to comment.
The complaint follows the copyright-infringement lawsuit filed by the Recording Industry Association of America (RIAA) in December 1999.
The RIAA suit alleged that key music companies stood to lose billions of dollars in royalties due to Napster's freewheeling music-swapping service.
Napster has been implementing a series of filters recently to weed out the service's copyright material, but some claim the site has not gone far enough in protecting copyrighted works.