PRINCETON, N.J. — Princeton University has joined a growing list of colleges that have deals to allow students to download music free — and legally.

The Ivy League School announced the arrangement Wednesday with Ruckus, a Virginia-based company that markets music solely to college students.

Under the deal, Princeton students can put all the music they want on their computer hard drives through Ruckus’s library of 1.5 million tracks. The songs are free because advertisers pay the company to get on the Web sites that students see.

But there’s a catch for the students: It will cost about $20 a semester to be able to put the tunes on their MP3 players. And the many Princeton students toting Apple’s iPod will be out of luck. The music won’t play on those popular devices.

Free music downloads are becoming widely available on campuses, partly as an effort to stem illegal downloading.

Ruckus is now available on 76 campuses across the country, including New Jersey’s Rutgers and Rider Universities. Add in other services, such as Rhapsody, Cdigix, and the now-legal version of Napster, and the number of campuses with free music is in the hundreds.

“It’s fantastic that you can listen to music for free on your computer legally, but it’s so typical that you can’t listen to it on an iPod,” said Byron Austin, a senior from Morristown told the Star-Ledger of Newark for yesterday’s newspapers.