WASHINGTON — Phoenix has overtaken Philadelphia as the nation's fifth largest city, underscoring decades of population losses in America's big industrial centers.

The nation's population has nearly doubled since 1950 — adding about 150 million people. But of the 20 largest cities at mid-century, all but four have shrunk, some by a lot.

Detroit, Cleveland, Pittsburgh, St. Louis and Buffalo, N.Y., have all lost more than half their population in the past half-century, according to estimates released Wednesday by the Census Bureau.

Utah's largest, Salt Lake City, has also seen a population drop. The city had an estimated population of 178,858 on July 1, 2006, a loss of 2,906 people since 2000. And other core cities in Salt Lake County also saw declines as smaller suburban communities continued to grow.

However, while Salt Lake City lost population from 2002 to 2004, it has been gaining population since then, and that growth is expected to continue, said Alison McFarlane, the city's senior adviser for economic development.

"Some of what we see in the numbers dipping a bit is we're a built city and the housing stock is aging," she said. "Now we're seeing the reverse ... the downtown rising, the City Creek project, (developments) are almost too numerous to name, we have so many projects now coming on line for housing."

In 2005, there were about 6,000 people living in the downtown area, and by 2010 that number is projected to reach 10,000, she said. And the city is beginning the master planning for its last undeveloped area northwest of the airport.

Meanwhile the estimates showed suburban areas continued to grow, as did St. George in Washington County, which since 2000 has moved from the state's tenth to eighth largest city, said Pamela Perlich, senior research economist at the University of Utah.

In Salt Lake County, West Jordan has surpassed Sandy as the state's fourth largest city, with an estimated population of 94,309 on July 1, 2006. Salt Lake remained the largest, followed by West Valley City (119,841) and Provo (113,984).

The growth is expected to continue in bedroom communities in southwest Salt Lake County and northwest Utah County, said state demographer Juliette Tennert. And some of those cities weren't even incorporated a decade ago.

Herriman, for example, grew by 33 percent in one year, she said. "That town wasn't even on the map 10 years ago, and it continues to experience really strong growth."

And McFarlane believes that Salt Lake City will be competitive as young families seek out homes.