Every weekday, Paul Matarrese leaves his house in Penn Hills, grabs an express bus to Downtown and then walks to his job as store manager at The Andy Warhol Museum.

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With that simple ritual, he contributes to a remarkable phenomenon -- an influx of commuters that gives Pittsburgh one of the biggest surges in daytime population of any city in the nation.

More than 180,000 suburbanites flow into Downtown, Oakland and other city neighborhoods each day to work, boosting Pittsburgh's population by 41 percent, the fourth-highest proportional "day surge" among large cities in the nation.

The clear national leader in this daytime differential, according to University of Pittsburgh researcher Christopher Briem, is Washington, D.C. The huge job engine known as the federal government helps pull in more than 480,000 commuters each day, expanding the district's population by 72 percent.

After the nation's capital come Atlanta (62 percent), Tampa (48 percent), and Pittsburgh and Boston (both at 41 percent).

The figures, released by the U.S. Census Bureau in October, are based on the 2000 Census, but the patterns they describe haven't changed, urban experts say.

Even though each of these cities has a unique story to tell, they share two common factors that heighten their daytime surge.

The first is that they are relatively small cities inside larger metropolitan regions.

The District of Columbia is constrained by historic boundaries; Boston and Pittsburgh, as older eastern cities, haven't been able to annex new territory for decades; booming Atlanta's last annexation came in the 1950s; and Tampa is hemmed in by water and the nearby cities of St. Petersburg and Clearwater.

The second common thread is that each city has managed to maintain a vibrant job market in its downtown and other city neighborhoods.

In Pittsburgh's case, the number of jobs actually has increased over the last decade, Mr. Briem said.

Jobs inside the city rose from about 300,000 in 1992 to 320,000 in 2001, the most recent year statistics were available, he said.

"It's pretty amazing that we have had employment in the city at the same level or higher than it was 20 or 40 years ago," Mr. Briem said. "That's an amazing juxtaposition when you look at the people we've lost" in city population.

The growth has been powered by employment at UPMC and the city's major universities, said Bernie McShea, a senior vice president at the Allegheny Conference on Community Development, the influential business-backed public policy organization.

The bustling back office operations of PNC Financial Services Group and Mellon, the addition of executive jobs at H.J. Heinz and Del Monte and the growth of software firms such as CombineNet in the Strip District and Vigilant Minds in Bloomfield also have abetted growth, he said.

And the jobs tend to be premier jobs, Mr. Briem added. In 2001, city jobs averaged nearly $40,000 a year in pay, compared with about $30,000 in the rest of the region.

David Rusk, an urban policy consultant based in Washington, D.C., said the top "day surge" cities "are typically small [area] cities at the heart of metropolitan areas that are growing, but where the city has a well established downtown core and major anchor institutions, like the universities or hospitals."

Pittsburgh differs from the other day-surge cities because its entire region actually lost population in the last census.

But the city lost residents at a much greater rate than the region as a whole -- in fact, Pittsburgh's decline accounted for 98 percent of the region's total population loss.

As a consequence, the proportional increase in the city's daytime population probably will grow in the future.

With so many suburbanites depending on Pittsburgh for their economic lifeblood, the question constantly arises: Are they paying their fair share for the services they get from the city?

The salaries that commuters earn contribute to the payroll taxes that Pittsburgh firms are paying to the financially-strapped city, but such big nonprofit employers as UPMC and the University of Pittsburgh don't have to pay that tax. The city also gets nothing from the suburbanites' real estate taxes.

That means the main burden on commuters is the occupation privilege tax that is applied to everyone who works in the city. It was raised in 2004 from $10 to $52 a year under the city's financial bailout plan.

City budget officials say the tax will bring in about $12.5 million a year, about four times as much as the old tax. But it won't come close to the $46 million that would have been generated by a proposed income tax increase that would have included commuters.

Pittsburgh Councilman Doug Shields, who chairs council's finance and budget committee, said he always thought a commuter income tax was "rather incendiary" and would have pitted suburban and city residents against each other.

A better solution, Mr. Shields said, would be a state revenue sharing plan that would compensate cities such as Pittsburgh for the high percentage of tax-exempt property they have. He acknowledged, though, that the chances of such a proposal getting through the state Legislature seem dim.

In the meantime, Pittsburgh can take some comfort from the fact that none of the other "day surge" cities have a commuter tax, either.

In fact, Mr. Rusk noted, when Congress gave the District of Columbia home rule powers, it prohibited the district from ever levying such a tax.

The top five "day surge" cities have one other thing in common: They all are trying to keep their business districts from emptying out again at night by promoting new downtown housing.

In Pittsburgh, the latest Downtown development plans call for about 850 housing units to complement new office and retail space.

Pittsburgh is a relative latecomer to this trend -- its plans are a far cry, for instance, from Tampa's proposal to add 6,000 housing units near its downtown.

All around the nation, said urban consultant and former Indianapolis mayor William H. Hudnut III, "there are three cohorts of people who are ripe for downtown living. One is the young, single laptop crowd who enjoy the amenities of urban living; the second is couples who don't have children; and the third is the empty-nester population that is tired of the commute and having the big house out in the suburbs. I call them the singles, the mingles and the jingles."

Mr. Matarrese, the Penn Hills commuter, would love to be a city resident again.

"I used to say I'd never live on the other side of a tunnel," Mr. Matarrese said, "but then I met someone who lived in Penn Hills, so here I am."

Still, he is a city soul, through and through.

"People ask me for directions all the time about how you get to some place or another in Penn Hills or Monroeville, and I say that I have no idea.

"I just get on that bus each day to come Downtown."