Don Imus's morning radio show is syndicated in more than 90 cities. Three hours of it are broadcast daily on cable TV's MSNBC. He draws headlines by embarrassing the president and first lady at the Radio & Television Correspondents' Dinner, which is repeatedly broadcast on C-SPAN. He appears on the cover of Newsweek, which proclaims him "Lord of the Air." For all the exposure and media coverage he gets, it would be natural to think that Imus is a popular radio personality in Washington.

Natural, but wrong.

In Washington--which ought to be a top audience for Imus, given his substantial emphasis on politics--he barely shows up in the quarterly Arbitron ratings. The most recent numbers were delivered to area stations Wednesday, and they displayed the customary middle- to bottom-of-the-pack finish for Imus, who is heard on sports/talk WTEM.

For all listeners in his time slot--6 to 10 a.m.--Imus ranked 20th of 27 stations surveyed (combining the simulcast signals of WTOP and WWZZ). Syndicated host Tom Joyner, who is heard on WHUR, finished first, with Howard Stern (WJFK) close behind. The morning shows on WGMS (classical) and WJZW (smooth jazz) finished ahead of Imus. He tied with the morning show on WAVA (Christian).

But that's not the best way to look at the I-man's ratings. Radio stations, and the programs on those stations, aim at very tight demographic groups. Radio is a leader in the media phenomenon known as "narrowcasting," or niche marketing.

WTEM targets men between 25 and 54, a powerful consumer group. Therefore what matters most is where Imus ranks among the other morning hosts who also aim at that demographic. If he scores high ratings in that category, WTEM can tell its advertisers that Imus is cleaning up among the listeners they most want to reach.

But he's not.

Among men 25 to 54, Imus tied for 15th with Les Brown (WMMJ). In this key category, Stern finished first by a landslide, followed by all-news WTOP. Also finishing ahead of Imus were Joyner, Russ Parr (WKYS) and Donnie Simpson (WPGC), Gary Murphy and Jessica Cash at country WMZQ, Dave Adler at Oldies 100 and Tim Brant and Andy Parks at news/talk WMAL.

What gives?

The conventional wisdom in radio is that high ratings mean more advertising dollars: The more people who listen to your station, the more you can charge to air commercials. In large cities, a 1 percent increase in listeners can equal a $3 million hike in annual revenue. And vice versa: It's not unusual for deejays and program directors to get fired over a slight dip in ratings from quarter to quarter.

In Boston, WEEI announced it would drop Imus in August. The station's program director proclaimed that the I-man had lost his edge, not to mention his ratings among 25-to-54-year-old men. The station figured Imus wasn't worth the $1 million it was paying him, according to industry sources.

But he was quickly snapped up by crosstown rival WBOS, albeit for less. In Boston--the nation's eighth-largest radio market--Imus's show often scored among the top five for men 25-54, which was good enough for WBOS.

Imus isn't even required to crack the top 10 in Washington, the nation's ninth-largest market. Here, Imus makes less than $500,000 a year, sources say, and is a moneymaker for WTEM, even with his low ratings.

There are any number of reasons for Imus's lack of popularity in Washington. First, he is on an AM station in an FM town. Washington leads the nation in FM signal penetration, but it is a lousy AM market. Even though WTEM has a mighty 50,000-watt signal, folks in Washington simply are not predisposed to tune to the AM band.

Also, Imus's typical listener is a white man around 47 years old. Imus, unlike Stern, has little crossover to black or young listeners. Then there is the issue of the Arbitron diaries, a shopworn excuse for anybody's low ratings. Bennett Zier, WTEM's general manager, says Imus's listeners may be less likely to fill out the ratings diaries and return them, which might mean there are more people listening to Imus than are accounted for.

So who exactly does listen to Imus? And how does he stay on the air in Washington?

"There's an old radio expression," Zier says. "Don't count the people who listen; the people that listen count."

Imus's guest list reads like a Who's Who of the media and political elite: "Meet the Press" host Tim Russert. New York Times columnist Maureen Dowd. Network news anchors Dan Rather and Tom Brokaw. TV reporters Cokie Roberts, Andrea Mitchell and Mike Wallace.

And then there are the pols: Bill Bradley. John Kerry. Chris Dodd. Bob Dole. John McCain. Al D'Amato. Even Bill Clinton went on Imus's show before the 1992 New York primary. Imus called him "Bubba."

Lump these names together and you've just about defined the term "opinion-makers." Imus's listeners are the tiny club of men and women who make or report on government decisions--and who all seem to know each other.

"Imus more than pays for himself in ad revenues," Zier says. "There's not a better guy to sell a computer or an F-14."

In other words, the members of a Senate subcommittee may not listen to one word of WTEM's sports programming, but they do listen to politicians and media folk on Imus's show, which is sandwiched around ads for defense contractors. Meanwhile, over on WJFK, Howard Stern is howling while a woman takes off her underwear in his studio.

Imus's ratings suggest that his Washington audience consists essentially of the people who appear on his show and the people they bump into on the Hill or at Georgetown parties.

This is a surprising evolution for Imus, who pioneered the shock-jock shtick in Cleveland and New York in the '70s and '80s. Nowadays you're more likely to hear him, at 59, soberly discussing politics or government than bad-mouthing the New York Knicks. He gets guests who would make Jim Lehrer drool. He makes bestsellers out of books on his reading list, books about culture and the media and public affairs. He is active in charitable foundations. His audience is wealthy, smart and well connected. All of which fits the exact profile of a radio host who might easily have a morning show on . . .

. . . public radio.