Since Monday, the Recording Industry Association of America, the main trade group representing record labels, has sent about 60 legal notices to the Internet-service companies that provide Web connections for "Open Napster" servers. These are computers that run Napster-like software, but aren't associated with the Redwood City, Calif., company's service.

Numerous free programs, such as one called "Napigator," can be downloaded from the Internet and used to locate Open Napster servers. Napigator author Chad Boyda said there are about 350 such machines operating, though the number varies daily.

Cary Sherman, general counsel for the RIAA, said the actions against the Open Napster movement were made possible by last week's decisive anti-Napster ruling by a federal appeals court, which settled the legal issues in the case overwhelmingly in the record industry's favor. The terms of an order that could lead to Napster's closure will be argued over next Friday during a hearing before U.S. District Judge Marilyn Hall Patel, who issued an earlier ruling in the case and was asked by the appeals court to craft an injunction.

The RIAA itself wasn't a plaintiff in the suit against Napster, a role played by the record labels individually. The industry group took action against the Napster clones because it serves as the labels' agent in tracking music piracy, routinely sending out notices against Web sites that host unauthorized recordings as well as the service providers they use to connect to the Internet. Under a 1998 law called the Digital Millennial Copyright Act, an Internet-service provider must, upon being notified, block access to one of its customers if they regularly infringe copyrights.

Boyda said the industry's move was expected. "We knew it was going to happen eventually, though it is coming sooner than people thought it would," he said.

Legal observers say the issues against the Open Napster servers are clear-cut, especially now that the appeals court has ruled that the Napster service abets copyright infringement on a massive scale. Boyda agreed that many of the organizations that received the RIAA notice had little choice but to comply with it, although he said a few may end up fighting it.

But he also noted that some of the servers are located overseas. In such cases, RIAA's Sherman acknowledges, it will be more difficult to enforce compliance with a shutdown request.

It is likely that smaller versions of Napster will continue to pop up on the Internet, especially since operating an Open Napster machine requires little more than a PC and a fast Internet connection. While these servers won't have anywhere near the global reach of Napster, they are likely to be a small irritant to record labels, and this week's "take down" letters mark the start of what is likely to be a lengthy cat-and-mouse game.

Besides the Open Napster computers, the record industry also must deal with Gnutella, a file-sharing system that doesn't rely on central computers and thus is much harder to shut down in court.

Gnutella has its own problems, chief of which is the fact that the current version of the software can't readily accommodate the millions of people that Napster does. Jordan Ritter, an early Napster engineer who is no longer at the company, estimated that a relatively small number of Gnutella users could, in theory, generate more network traffic than the entire Internet could handle. Gnutella's developers say they are working on that problem. Meanwhile, Sherman said his group had ideas about ways of dealing with Gnutella, but wouldn't discuss them publicly.

Earlier this week, Napster unveiled a proposal under which it would pay the record industry $200 million a year if it is allowed to stay open. But record industry officials have been cool to the idea, and say they are moving ahead with their own online plans that don't involve Napster.