New York ? When several Yellow Pages publishers came up for sale recently, something interesting happened: Investors paid huge sums for them.

The Yellow Pages, possibly the least glamorous backwater of the media business, is one of the most profitable. Sure, the Internet is a long-term challenge, but the business model of selling ads to local plumbers and pizza joints still has an amazing staying power.

People use the Yellow Pages less frequently than they did before the Internet arrived, but that decline has leveled off in recent years, and more than half of all adult Americans still use the Yellow Pages at least once a week, down only slightly from a decade ago, according to Knowledge Networks/SRI, a research group.

Small businesses still spend mountains of money to advertise in them. Like classified advertising in newspapers, Yellow Pages are one of the only forms of what’s called “directional advertising,” the kind that points consumers to a vendor when they’re just about to buy something.

Naturally, small businesses like florists and hardware stores love this.

“It’s an enormously effective form of advertising,” said Arnie Ursaner, an analyst with CJS Securities in White Plains, N.Y. “If you’re a restaurant or a plumber, this is where you go.”

For Lee Portokalis, owner of a medical supplies store in Chicago, advertising in the Yellow Pages remains an extremely effective way of attracting new customers. One publisher provided him with a special number to put in the directory so calls coming in from Yellow Pages users could be tracked by computer.

He found that at least 100 people call every month, making it more than worth his while to spend the $10,000 a year to advertise in the directory. “People are definitely using these books,” Portokalis said.

“I use the book myself when I need something at home, like getting my floors fixed,” Portokalis said. “I’ll call around a few places to get some estimates. That’s not something you can really do on the Internet.”

Despite competition from the Internet, investors are learning to love the Yellow Pages. Several phone companies have put their directories up for sale in recent months, seeking cash to pay down debt, and investors have been willing to pay top dollar.

Lee Portokalis, owner of a medical supplies store in Chicago, holds a copy of his ad in the Yellow Pages. Portokalis says at least 100 people call his business thanks to the ad every month, making it more than worth his while to spend 0,000 a year to advertise in the directory.

Last month, Yell Group PLC, which publishes directories in the United States and Britain, raised $2.1 billion in a blazing debut on the London Stock Exchange, and in June the main Yellow Pages company in Italy sold for $6.6 billion. Last fall, investors including the Carlyle Group, home of former Securities and Exchange Commission chairman Arthur Levitt, paid $7 billion for Qwest’s directory business.

Investors are finding out what phone companies knew all along: Phone directories are staggeringly lucrative. With profit margins of 50 percent or more, they vie with local TV stations as some of the most profitable media businesses.

At $14 billion a year, the advertising spent on Yellow Pages is larger than all of local TV advertising, and about the same size as the magazine industry. Plus, since the advertisers are local businesses, they’re less likely to pull out in an economic downturn as national brand advertisers do.

However, the Internet has been taking its toll. The frequency of usage of Yellow Pages has been declining, from a total of nearly 20 billion lookups per year in 1994 to about 15 billion last year, according to the Yellow Pages Integrated Media Assn., a trade group.

But that usage has been essentially stable since 1999, suggesting the damage from the Internet has been done — that is, until looking up phone numbers on the Internet is always easier, more reliable and accessible than reaching for the phone book on the kitchen shelf.

“This industry isn’t in immediate danger of going away, but it clearly faces challenges,” said Charles Lachlan, an analyst with The Kelsey Group, a research firm specializing in the directory business. “Until a real replacement emerges, the Yellow Pages will be a stable business.”