Heavy metal band Metallica has sued the Napster MP3 trading software company and a number of universities, claiming that they are responsible for violations of the band's copyright.

It is the second time that Napster, which markets music-swapping software, has been in court. However, it is the first time that US universities have been caught up in such a legal wrangle.

This week a German court ruled that America Online and other ISPs are liable for pirated music traversing their systems in that country. As yet, such a ruling has not been introduced in the US.

Napster's software enables music fans to open pieces of their personal hard drives to everyone using Napster, sharing whatever MP3 tracks they have already downloaded or stored. At any time, thousands of people are online, sharing hundreds of thousands of tracks, many of which are technically illegal to download without the permission of the copyright holders.

The Recording Industry Association of America has already sued Napster, contending that the company allows copyright violations by facilitating the easy exchange of music. The decision on this case has been postponed.

Napster maintains that it is not hosting any of the music and says that it is protected by US federal copyright provisions that shield ISPs from being liable for illegal material sent over their services.

The US universities cited in the lawsuit include Yale University, the University of Southern California and Indiana University. All three universities have not blocked the use of Napster.