(FORTUNE Magazine) – What's the most valuable show on television? Not ER, prime time's top-rated program. Not Seinfeld, the sitcom whose stars got GE stock options for returning to work this season. Need a clue? The truth is out there.

The X-Files, the five-year-old science-fiction phenomenon, has created an industry worth more than a half-billion dollars, demonstrating the beauties of synergy. Fox's highest-rated show, produced by the Twentieth Century Fox studio, has brought in cash from network advertising revenues, cable and syndication deals, foreign sales, and merchandising. Next summer the show will spawn a theatrical movie. But if News Corp. CEO Rupert Murdoch hadn't owned both a studio and TV network, the show might have died prematurely.

That's because The X-Files took time to catch on, ranking a dismal No. 111 in the Nielsens at the end of its first season. Any other network would have canceled it. But fledgling Fox, desperate for a hit, believed the show's cult following could be broadened. Now no one questions its worth. "The X-Files is probably the biggest single asset that Fox television has," says NBC's CEO, Bob Wright.

Just count the ways The X-Files fills News Corp.'s coffers. First, the Fox broadcast network collects nearly $5 million in advertising revenue each time the show is run. Reruns have been sold to local TV stations and to the Fox-owned cable network FX, which has enjoyed dramatic growth in ratings and improved distribution since getting X-Files. "The show has put us in a position where consumers are asking for FX, and cable operators are coming to talk to us about the network," says Mark Sonnenberg, executive vice president of FX. Internationally, the show is flourishing too, spooking viewers on the BBC and Murdoch-owned BskyB in Great Britain, as well as in Germany and France.

The biggest payday may still be ahead. With a presold audience of 22.7 million weekly X-Files viewers, this summer's movie could do as well or better than recent TV spinoffs like The Brady Bunch and Mission: Impossible. "There's an X-Files audience that lives and dies with the show," says Fox Television CEO Chase Carey.

--Henry Goldblatt