WASHINGTON -- Boston has lost more than a quarter of its residents since the middle of the last century, according to estimates to be released today by the Census Bureau.

Boston is among 16 of the country's 20 largest cities in 1950 that have shrunk, some by a lot. This has happened while the nation's population has nearly doubled, adding about 150 million people.

Boston lost 210,681 people -- to end up with a population of 590,763 last year, according to estimates by the Census Bureau, which is releasing 2006 population estimates for US cities. The Associated Press compared those estimates with population totals from the 1950 census.

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

Detroit, Cleveland, Pittsburgh, St. Louis, and Buffalo. have all lost more than half their population in the past half-century.

Philadelphia lost nearly a third of its residents, slipping to about 1.4 million people last year, according to estimates being released today by the Census Bureau.

Like many big cities in the Northeast and Midwest, Philadelphia has suffered through a decline in the nation's manufacturing economy. City officials, however, have vowed to rebound.

"Philadelphia is not going to disappear," said Gary Jastrzab, deputy executive director of the city planning commission. "We have a good quality of life here. We have major universities, major health facilities, and a very active pharmaceutical industry."

Phoenix was barely in the top 100 cities in 1950; it ranked 99th, with about 107,000 people. Last year, it had 1.5 million.

Phoenix added 43,000 people from 2005 to 2006, more than any other city, according to Census Bureau estimates.

It was followed by San Antonio; Fort Worth, Texas; Houston; and North Las Vegas, Nev.

New Orleans, which is struggling to rebuild following Hurricane Katrina, lost the most people, about 228,000. The Census Bureau estimated the city's population at 223,400 last year, a little less than half its size before the storm.

Americans have been migrating south and west for decades in search of better jobs and warmer climates. They have also been moving to the suburbs and beyond, in search of bigger yards and houses, lower crime rates, and better schools.

In 1950, nearly a fifth of the population lived in the nation's 20 largest cities. In 2006, it was about 1 in 10.

Many older cities are trying to reinvent themselves, relying on the universities, health centers and cultural attractions that have long been desirable, said William Frey, a demographer at the Brookings Institution, a Washington think tank.

"It used to be that the city was the whole regional economy. Now, it is just the center," Frey said. "These cities certainly can be viable with smaller populations."

© Copyright 2007 Globe Newspaper Company.