Population in the city decreased from 1,517,550 to 1,449,634 in the seven years, a loss of nearly 68,000 people, according to Greg Harper, a demographer for the bureau.

That drop of 4.5 percent represents the largest percentage loss in population of a top-25 U.S. city between 2000 and 2007, figures show.

Viewed more broadly, the population of the Philadelphia region as a whole has grown even as the number of people in its urban center has declined. The populations in the counties surrounding the city have either remained stable or increased, though experts say these changes are not necessarily linked to the city's loss.

The Philadelphia Metropolitan Statistical Area, which includes South Jersey, parts of Delaware, and Cecil County, Md., had a population gain of 139,000 (2.5 percent) between 2000 and 2006, according to the Census Bureau.

In the bureau's latest analysis, Philadelphia's rate of loss exceeded those of Detroit (3.6 percent), Baltimore and Chicago (both 2.1 percent). Conversely, Boston and New York increased in population over the seven years, at the rates of 1.7 percent and 3.3 percent, respectively.

"Philadelphia has been losing population for a while," Harper said. "It's not a new trend."

Between 1990 and 2000, for example, the city lost 68,027 people, the third-highest number among 243 cities with populations of more than 100,000, Harper said.

"That's pretty consistent," Harper said. "Philadelphia is one of the fastest-losing cities in the country."

Last year, Phoenix supplanted Philadelphia as the fifth-largest U.S. city, with a population of 1,552,259.

Philadelphia's population peaked at 2,071,605 in 1950, and has dwindled ever since. The city's current population is the lowest since the start of the 20th century, when the number of residents increased from 1.3 million in 1900 to 1.6 million in 1920, figures show.

Trying to make sense of the latest population estimates, local experts offered differing analyses. "It's not an easy question," said David Elesh, a Temple University sociology professor and an expert in urban development.

One thing is certain: It's not as though people are dying off. There were 38,000 more births than deaths in Philadelphia between 2000 and 2007, Harper said.

That net loss of nearly 68,000 means that "more people were moving out than moving in," Harper added.

Elesh's Temple colleague David Bartelt said a major reason for the loss was that the city had been slow to develop more nonmanufacturing jobs to keep population and attract new people.

"If we had a better job base, we wouldn't have people moving out," said Bartelt, a professor of geography and urban studies. "We need a nonmanufacturing job base that pays better than tourism jobs, which pay poorly."

Lately, the city has been attracting young people, especially to Center City, which is experiencing a population boom. But, Bartelt said, it's not enough. "We do a terrific job of inviting students to study here," he said, "but a not-so-terrific job of keeping them here."

Bartelt cited Boston as an example of a city that had a manufacturing past but that consciously began in the 1980s to become more high-tech, which kept the young people who attended its many colleges and universities.

"That's what people mean by 'the miracle of Boston,' " he said. "We had the same raw materials as Boston, but didn't create the significant program of business development that they did."

Interestingly, the people who are leaving Philadelphia aren't necessarily going into the suburbs, Bartelt said. Although analysis by Sue Copella of the Pennsylvania State Data Center in Harrisburg shows that population rose in the counties of Chester (12.2 percent), Delaware (0.4 percent), Montgomery and Bucks (both 0.3 percent) between 2000 and 2007, Bartelt's research shows the suburbs are not absorbing the city's exodus.