"Nearly three-fourths of the population growth statewide is due to our children and grandchildren," Spendlove said. "Migration puts us over the top."

Perlich said many of those new children are born to immigrants, who tend to have a higher birthrate than the state's established population.

"The point is, who is having those children," she said. "A lot of the record-breaking births are actually children of immigrants."

Nationally, Flagler County, Fla., located between Daytona Beach and Jacksonville, topped the list of the nation's 100 fastest-growing counties, with an estimated one-year growth of 10.1 percent to 69,005 people.

The nation's largest county was Los Angeles County, with an estimated population of 9.9 million.

In Utah, Tooele County, which has been the state's fastest-growing county over the past few years, continued to show strong growth.

The county's population has boomed by 22 percent since 2000 to an estimated 49,688 in 2004.

Nicole Cline, Tooele County's planning and economic development director, said people are attracted to the county because of its traditionally low housing costs and large lots. While many residents commute to Salt Lake City, the county is working to develop its economy — a Wal-Mart distribution center recently added 600 jobs, she said.

Other counties along the Wasatch Front showing strong one-year growth were Summit, which grew by 3.2 percent to 33,843, and Wasatch, which had an estimated 18,139 people in 2004, a growth of 2.9 percent.

The state's largest county, Salt Lake, ranked 41 in size nationally with 935,295 people in 2004, according to the census estimates. The census shows Salt Lake County has gained nearly 35,000 people since 2000 — the largest numerical gain of any county in the state.

Salt Lake County gained an estimated 10,535 people last year, a growth rate of 1.1 percent.

Utah and Davis counties were the other counties with the largest numerical gain. Utah County, with an estimated 2004 population of 403,352, gained 6,182 residents in one year, and Davis County grew by 5,900 people to an estimated 261,208.

Washington County, increasingly tied to Nevada's economy, grew faster last year than Clark County, Nev., where Mesquite and Las Vegas are located, though in straight numbers Clark County, with a population estimated at nearly 1.7 million, saw a far greater numerical gain.

With its warm climate and access to I-15, Washington County's growth is geographic in nature, Spendlove said.