So you just downloaded an MP3 version of the latest Jennifer Lopez single from Gnutella or one of the other free file-sharing networks. You're not feeling too guilty, and in any case, no one will ever know, right?

Don't count on it. Lopez's recording label, Epic Records, is owned by Sony Music Entertainment, and Sony is one of a number of media giants hiring the services of a new breed of content-tracking firms to combat digital piracy. These online private eyes are using the latest digital fingerprinting technology to scan public computer networks for unauthorized copies of music files, still images, movies and software. And they can watch as those illicit files spread from hard drive to hard drive-whether or not the files bear the invisible digital "watermarks" often used to identify their original owners.

At San Jose, CA-based BayTSP, for example, software engineers have developed a system that uses the unique characteristics of a client's copyrighted content-amplitude and frequency in an audio file, for example-to extract a digital signature. The system then patrols the entire Internet, including the major file-sharing networks, scanning key sections of five to ten million files per day for matching signatures and automatically e-mailing infringement notices to offenders and their Internet service providers. Because the system doesn't depend on embedded information to find copies, it can be used to locate files that may have been stolen years before the invention of watermarking. "You can give me a piece of pirated content that's been on the Internet for five years, and I can still tell you what it is and where it's being distributed on the Internet," says Mark Ishiwara, BayTSP's chief executive and technology officer.

BayTSP has competitors in the digital-policing market: Arlington, VA-based Cyveillance uses technology similar to BayTSP's to track pirated corporate logos and sensitive business documents, while Seattle-based Loudeye Technologies and Alexandria, VA-based Relatable use the technology to help music companies gauge how widely their songs are being traded. But BayTSP clients such as Adobe Systems, the San Jose-based maker of graphic-arts and document-sharing software, say the company's help in sniffing out and confronting pirates is quickly becoming indispensable. "It gives us an idea of how much pirated product is out there and who's using it," says Cynthia Navarro, manager of Adobe's antipiracy programs.