Loudoun County's seven towns have continued their swift population growth but have lagged behind the county as a whole, according to U.S. Census Bureau figures released last week.

Between April 2000 and July 2004, growth in the towns ranged from 22.8 percent in Hamilton to 33.2 percent in Middleburg, according to the census estimates. In the same period, the county's overall population grew by 41 percent, census figures show.

But some county officials questioned those numbers, saying they either overstated or underestimated actual population growth in the towns.

Middleburg, famed for trying to temper the effects of Loudoun's boom, saw its population rise faster than any other town -- from 632 in 2000 to 842 last year -- according to the census estimates.

But Claudia W. Dornin, assistant town administrator, said she was "somewhat skeptical" of that conclusion. She said, however, she doubted county figures as well. In a preliminary analysis sent to Mayor C.L. "Tim" Dimos, Dornin noted that county demographers, using building-permit data, put Middleburg's population at 641, significantly lower.

"The County's estimate is likely too low because it does not account for churn in housing (e.g. elderly couple to family unit)," Dornin wrote. "I expect the true population falls somewhere between 641 and 842 (closer to 750??)."

Dornin said she was looking at ways to get a "more realistic" estimate, which could include using figures from a countywide school census due later this year and by polling water utility customers.

Whatever the precise figures, Dimos said, Middleburg's growth has been unfolding as anticipated and "does not cause me alarm. . . . It's under control. It's what we have planned for. That's good."

Middleburg is growing more slowly than the county as a whole, he said, because there's "very limited physical space" for new housing.

Proponents of preserving western Loudoun's rural character have a responsibility to accommodate some so-called in-fill development, meaning projects on land in town near existing homes and shops, Dimos argued.

"I think in the towns, and Middleburg in particular, we need to contribute what we can toward realizing our goal of saving open space," Dimos said. That means officials should "permit, if not encourage, in-fill development so it's not spread out hopscotch all over the countryside," he said.

But officials also said they were concerned about a potential surge in growth outside their town limits -- and their control. They cited debates over whether county officials should approve numerous housing projects in an area west of Dulles International Airport known as the transition zone and over the future of the county's rural zoning.

"There's a negative impact on us, on quality of life, whether the growth is inside the town or not," said Purcellville Town Council member Robert W. Lazaro Jr. "We get all the traffic and congestion that comes with the development [outside town]. We're left fixing the roads. We're left having to hire more police officers," he said.

Purcellville's population increased from 3,584 in 2000 to 4,479 last July, according to the census estimates. But Town Manager Robert W. Lohr Jr. said the census undercounts the town's population. "We're showing some dramatically different numbers," he said, adding that the latest census figures missed about 1,000 residents. The town uses occupancy-permit data and water-hookup numbers to count residents, he said.

Loudoun County demographer Clark Draper said the county uses building-permit data to come up with its town population estimates, while the census estimates include assumptions about such factors as migration patterns. The census methodology can sometimes muddle local results, he said.

For instance, Draper noted, the census numbers show that population increased by 24 over four years in Hillsboro, Loudoun's smallest town. That would assume construction that has not occurred, Draper said.

"To go from 96 to 120, I don't know how they could possibly do that," Draper said. "We're not going to change the population of Hillsboro until we get some more new construction."

Draper said Loudoun officials consider a variety of population estimates to get a clearer view of the fast-changing county.

"We know Loudoun better than the Census Bureau does, and we're more comfortable with our numbers. But it's good to have another data source," he said.

In a photograph from 2001, propane tanks destined to serve new homes are kept in readiness at the site of a Purcellville housing development.Middleburg's population growth exceeds that of all other Loudoun towns, census figures say.