Here is how this contextual advertising has traditionally worked: Google and Yahoo post ads on hundreds of thousands of Web sites, but both operate as blind networks — they do not tell advertisers which sites their contextual ads run on. Instead, the advertisers buy keywords for ads across Google and Yahoo’s vast networks of Web sites, including the home pages of big media companies and the smallest of bloggers.

It would be akin to an advertiser buying space on 100 national television programs, but not being told when the ads ran. Google and Yahoo argue that because advertisers only pay when the ads are clicked on, that more specific information is irrelevant.

Nor has Google allowed advertisers to bid for keyword ads on specific Web sites — say, ESPN.com. Quigo says blind network buying lowers the prices that premier Web publishers receive because advertisers bid expecting an average site rather than a well-known, desirable one.

“Google, Yahoo and most other blind networks sit in the middle and own the advertiser relationships,” said Henry Vogel, the chief revenue officer of Quigo, which was founded in Israel in 2001. “By outsourcing their performance marketing programs to them, publishers get a check but little else. They don’t really build any longer-lasting strategic assets.”

Both Yahoo and Google play down Quigo’s inroads into the business. Emily Fox, a spokeswoman for Yahoo, said her company was not aware of losing any contextual text ad clients to Quigo other than ESPN. Kim Malone, director of online sales and operations for Google AdSense, said Google did not worry about Quigo.

“The David-and-Goliath story is always a great account,” Ms. Malone said, “but I think in this case, it’s just not accurate . We have a number of large publishers who have tried out other solutions, and they always come back.”

In response to further questions about Quigo, though, Google said it was prepared to make changes to its AdSense service that mimicked Quigo’s approach, an unusual step for a company accustomed to mapping the terrain in every aspect of its business.