Unity Entertainment has filed a copyright infringement suit against MP3.com.
The new suit came days after the company ended its long-standing battle with the major record labels. On Tuesday, it was ordered to pay $53.4 million to Universal Music Group to settle charges of copyright infringement.
A spokesman for MP3 said the company is "disappointed" to receive the complaint given that the firm has now signed licensing agreements with each of the five major record labels.
Michael Robertson, chairman and CEO of MP3.com, said: "We continue to firmly believe that the My.MP3.com and its technology infrastructure serves to empower artists and gives consumers the benefit of listening to their own music anywhere and at anytime."
Robertson said his company would respond to the suit "appropriately", but declined to say how or when they would do so.
MP3.com's Web site is host to what company officials believe is the largest online digital-music library available to consumers. The company says it has over 698,000 songs and audio files from 109,000 artists and record labels.