NEW YORK (FORTUNE) - Flip on Univision or Telemundo on any given night, and in between the melodramatic telenovelas and campy talk shows, you'll see the biggest consumer brands in the world pitching their products in Spanish. It would seem every major company, from car makers to fast food franchises, is trying to reach the nation's fast-growing Hispanic population, now more than 40 million strong.

But marketers looking to reach a lucrative swath of the U.S. Hispanic population may be speaking the wrong language. It turns out that much of the $4 billion spent annually on Hispanic advertising targets individuals born outside the United States, yet the majority of Hispanics in the United States – some 22.3 million – were born in the states.

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And even among foreign-born Hispanics, many millions are highly acculturated, which means they are fluent in English and Spanish, and identify strongly with both American and Latino cultures.

And the current crop of ads on Spanish-language television, radio and newspapers don't really resonate with bicultural Hispanics, says Jaime Fortuno, managing partner of Azafran Advertising, a New York-based unit of Puerto Rican agency Lopito, Ileana & Howie Inc. For starters, bicultural Hispanics may not read the local Spanish-language paper or watch Univision, with its heavy rotation of programming from Mexico. "This group is not about nostalgia for the home country," Fortuno says.

On the other hand, acculturated Hispanics aren't feeling the love from mainstream advertising, either, which still features few Latino faces and rarely features the kinds of scenes and story lines that try to capture Hispanic daily life.

So what kinds of ads work for a group Azafran has dubbed "the Fusion Market"? Multicultural marketing experts give high marks to a bilingual ad Toyota ran during the Super Bowl earlier this year to promote its 2007 Camry Hybrid vehicle. In the 30-second spot, developed by Conill (Toyota's Hispanic ad agency), a father speaks to his son in Spanish and English, drawing comparisons to the car's ability to switch from gas to electric power.

Azafran next month will unveil an ad campaign for one of GlaxoSmithKline's skincare products that features Hispanic women bussing each other on both cheeks – a familiar greeting in Latin cultures. Fortuno says the program, which can run in English or Spanish, was designed with the bicultural Hispanic woman in mind.

"We could have used a commercial from Mexico and it would have been just like home," he says. "But when you talk to women who live here and who have full knowledge of English, they want to hear messages that reflect the reality of their lives in the U.S."

Azafran and other advertising agencies pushing companies to market to bicultural Hispanics face some big challenges. TV and radio ad budgets are shrinking as marketers increasingly look at new media such as the web as ways to reach consumers. Amid such belt tightening, it would be pretty easy for a chief marketing officer to rationalize that she's reaching English-speaking Hispanics with her mainstream ads – or capturing those who prefer to communicate in Spanish through a traditional Spanish-language campaign.

Fortuno and his peers are asking that marketing executive to devote scarce resources to a third kind of niche marketing – without a lot of data to show how well it will fare.

Fortuno points out that other industries - the food and music businesses, for example - already have captured the fusion market with products that combine American and Latin culture. One of the hottest new music genres, reggeaton, fuses hip-hop with Latin beats and lyrics in English and Spanish.

Indeed, many entertainment executives predict Latino culture will increasingly work its way into mainstream American popular culture, much the way hip-hop has influenced everything from music to television to film. And so by reaching out to bicultural Hispanics now, marketers could end up with an even more valuable resource: An insight into what will eventually become the new mainstream.