The Recording Industry Association of America said its member companies filed suit against two students at Rensselaer Polytechnic Institute and against one student each at Princeton University and Michigan Technological University.

The RIAA has actively used the courts to pursue digital music pirates after a 9 percent dip in CD sales in 2002 that it blames for the most part on online file sharing.

In a statement, the association compared the file-sharing systems, which are open only to students on the universities' internal networks, as miniature versions of Napster -- the software and network that led to the explosion of music file swapping.

The four networks were offering nearly 2.5 million files, it said, including more than 1 million files on the largest network alone.

The complaints ask for the legal limit on damages in such cases, $150,000 per each copyright infringed.

The defendants named in the complaints are Daniel Peng at Princeton, Joseph Nievelt at Michigan Technical, and Jesse Jordan and Aaron Sherman of Rensselaer.

None of the four could be immediately reached by phone or e-mail. A personal Web site listed for Sherman on the Rensselaer site was not loading as of Thursday afternoon.

Last month the RIAA sent letters to 300 U.S. companies, across a variety of industries, warning them of specific evidence of illegal music swapping on their networks and the potential legal consequences of allowing it to continue.

The RIAA represents the world's major music companies, including Vivendi Universal, Sony Corp., AOL Time Warner, EMI Group Plc and Bertelsmann AG