That made Napster the fastest-growing software application ever recorded by the Internet research company.
From 1.1 million home users in the United States in February, the first month Media Metrix tracked the application, Napster use rocketed to 4.9 million users in July.
That represents 6 percent of U.S. home PC users who have modems, said Media Metrix, which pays people to install monitoring software on their computers.
It estimates total usage from a panel of about 50,000 people in the United States.
Napster was also used at work by 887,000 people in July, Media Metrix said.
Napster Inc. has been sued by the recording industry for allegedly enabling copyright infringement. The federal government weighed in on the case Friday, saying the service is not protected under a key copyright law, as the San Mateo, Calif., company claims.
Bruce Ryon, head of Media Metrix's New Media Group, said Napster was used by "the full spectrum of PC users, not just the youth with time on their hands and a passion for music."
The Napster program allows users to copy digital music files from the hard drives of other users over the Internet.
Napster Inc. said last week that 28 million people had downloaded its program. It does not reveal its own figures for how many people actually use the software.
Because the program connects to the company's computers over the Internet every time it is run, Napster Inc. can track usage exactly.