By NEIL STRAUSS The New York Times

The song may sound familiar at first, thanks to the unmistakable guitar riff from Nirvana's classic "Smells Like Teen Spirit."

But, suddenly, the recording changes course when, instead of the gravelly voice of Kurt Cobain, the smooth R&B harmonies of a Destiny's Child hit appear on top of the grunge music. As the recording moves on, it is clear that the song is neither fish nor fowl; it is a crossbreed that neither band ever intended, or even dreamed of.

It is something that is completely different, often illegal and, thanks to the Internet, becoming explosively popular.

Songs like this one, which combine different hits without adding any original music, may represent the first significant new musical genre to be lifted out of the underground, developed and then spread, mostly via the Web. The songs, called mash-ups or bootlegs, typically match the rhythm, melody and underlying spirit of the instrumentals of one song with the a cappella vocals of another. And the more odd the pairing the better.

The music industry has greeted them with mixed response. A radio station in London playing a popular mash-up with Christina Aguilera belting her hit "Genie in a Bottle" over the retro-rock of the Strokes was served with a cease-and-desist order by Ms. Aguilera's publisher, Warner-Chappell.

On the other hand, in Britain last week, Island Records released a legal mash-up, which entered the pop charts at No. 1. It combines music from three different artists - the new-wave icon Gary Numan, the R&B singer Adina Howard and the girl-pop group the Sugababes.

The music - there are hundreds of such recordings - is particularly popular in Europe, where D.J.'s play mash-ups at parties. But through the Internet it is spreading not only there but also in the United States. There are so many bootlegs using Eminem and Missy Elliott songs (Missy mixed with the 80's group the Cure, Eminem with the fey pop of the Smiths, and Missy with the heavy metal group Metallica, for starters) that some practitioners refer to making a bootleg as "doing a Missy" on a song.

The growing scene is a result of two technological forces that have been revolutionizing music-making and the record business: cheap computer software, which makes it possible for a teenager with no musical knowledge to create professional-sounding productions at home, and Internet file-sharing services, which provide a quick way to gather and share music. Naturally, the music industry is concerned about this, because in most cases the tracks are being used without permission.

But, today, when the Internet seems to loom larger in many music fans' heads than lawyers' threats, bedroom musicians on both sides of the Atlantic are undeterred. All they need to do is download or buy software programs like Acid (which automatically synchronizes the rhythms of different tracks). Then they can scour a file-sharing service for a cappella versions of songs, which record companies sometimes include on promotional singles for club disk jockeys. Using a program like Acid, they can combine their source material into a new song.

Afterward, the creators upload their musical patchworks back onto the same file-sharing service they grabbed the source material from.

The mark of a good bootleg, fans say, is that it doesn't sound at all like one song superimposed on top of another, but a new song in itself. Among the most popular bootleg artists are Freelance Hellraiser (responsible for the Aguilera mix), Osymyso (who combines more than 100 songs in one mash-up), Kurtis Rush and Richard X. The more popular acts create their music through sampling their own records and then spread the mash-ups through white-label (i.e. anonymous) singles or playing them on the radio. But the music can also be accessed on file-sharing sites like Kazaa and Audiogalaxy.

"If you take two or three or four great records and mix them together, you should end up with a superior product," said Steve Mannion, a co-editor at Boom Selection (www.base 58.com), a Web site dedicated to documenting the do-it-yourself remix, bootleg and sampling movements. "The best bootlegs don't sound like bootlegs; they work at a profound level, and actually sound like they are the original record."

Completing the circle back to the record store, an illegal CD collecting the years best mash-ups, "The Best Bootlegs in the World Ever," recently appeared on the shelves of some underground music retailers in England and the United States. It was created by profiteers who simply downloaded the songs from a file-sharing service and then burned them onto a CD. "It is a case of bootleggers bootlegging bootlegs," said David Dewaele, who, with his brother, Stephen, make up one of the most accomplished and long-standing teams, known alternately as 2 Many D.J.s and the Flying Dewaele Brothers.

Last year, the Dewaele brothers, Belgians who also play in the popular rock band Soulwax, created a legal mix album, but not without a lot of difficulty. It took the brothers two weeks to make the album, released as "2 Many D.J.s: As Heard on Radio Soulwax Pt. 2" - there is no Part 1 - but nine months to license the music (which includes songs by Dolly Parton, Sly and the Family Stone, and many others). And, even then, they were only able to clear the music on the CD for release in Belgium, Luxembourg and Holland.

Pirated copies of the album have been circulating in the United States, and some music executives who have heard it cite it as not only the remix album of the year but the best album of any kind released so far this year.

"It's my favorite record of the year so far," said Steve Greenberg, a former Mercury Records executive who now runs the independent label S-Curve Records. "It looks at music in a fresh and original way, and breaks down walls in ways that are particularly exciting considering how categorized and fragmented music is at the moment."

One of the Dewaele brothers' first mash-ups was a combination of the rapper Skee-Lo's light-hearted "I Wish," Survivor's anthem "Eye of the Tiger," and the Breeders's rock song "Cannonball."

From the opening track of their album, there is a distinct style and aesthetic at work. Often, the songs are cut up by computer, so that an introduction can be shortened, a verse removed or a section repeated to maintain the set's fast pace. "It has to be something that has some sort of edge to it, something weird that makes you go, `What is this!' " said David Dewaele.

Making new songs out of existing works, of course, is nothing new. There are precedents in everything from 20th century classical to cartoon music, and it is the cornerstone of hip-hop, be it early pioneers like Grandmaster Flash or later innovators like Dr. Dre. In the 80's and 90's, avant-garde sound artists like Plunderphonic, Negativland and the Tape-Beatles (as well as the pop pranksters the KLF) challenged copyright law with collages made of everything from found sounds to top 40 hits. But many musical observers trace the official beginnings of the British bootleg scene to the Evolution Control Committee, which in 1993 mixed a Public Enemy a cappella with music by Herb Alpert.

Today, there is a glut of such artists, and Mr. Mannion said that his Web site, Boom Selection, may receive as many as dozen new ones a week. Does that make it a fad or something here to stay?

"I dont know what will happen next," Mr. Mannion said. "When people hear this stuff so much, they can get bored of it. But to me, I'll never get bored with this stuff, because that's like getting bored of music itself."