The city’s population is growing at a slower rate than that of urban centers in the nation’s booming Sunbelt, but the Big Apple is outpacing its historical rivals in the Northeast and Midwest, data released yesterday by the Census Bureau show.

The five counties that comprise New York City grew by about 1.65% between 2000 and 2005 and stand at about 8.16 million people, according to the bureau’s estimates. While New York beat out counties serving cities such as Philadelphia and Chicago, which showed declines, the five boroughs lagged well behind hot spots in the South and West, including Los Angeles, Phoenix, Houston, and Miami.

The data further show that New York’s population declined slightly between 2004 and 2005 and that immigration was lower than the five-year average, a drop-off the city is disputing.

Analysts point to a variety of factors to explain the city’s growth rate. A steady flow of immigrants has kept New York ahead of other Northeast cities, while steep housing costs may keep the city from further growth.

New York has long relied on an influx of foreign immigrants to replenish losses caused by the departure of native-born residents. Long-range population trends are notoriously difficult to predict, but signs that immigration to the city may be slowing are a concern for future growth, urban studies and population scholars said.

“The trend is definitely slowing down and possibly plateauing, but where it is going it’s too early to say,” a senior fellow at the Manhattan Institute, E.J. McMahon, said of the city’s population. “The thing that’s pulling it down is a slowdown in immigration, as well as real estate and housing costs.”

A fellow at the Center for an Urban Future, Joel Kotkin, said immigration patterns are becoming more dispersed across America. “What I do sense is that immigrants are getting smart about America and are realizing that there are a lot of other places to go besides Los Angeles and New York,” he said.

Any drop in the city’s population would appear to cast doubt on the city’s preparations for a major population boom in the coming decades. Over the last several years, the Census Bureau’s release of a county-by-county population estimate has prompted an annual clash between the federal agency and administration officials who argue that the bureau consistently shortchanges the city’s population. The city contends that the bureau’s methodology does not take into account new housing construction, which is the basis for the administration’s calculations.

The bureau acknowledges that its methodology may not work for counties like New York’s. “It is something that we’re looking at, but we don’t have a definite plan as of yet to incorporate the housing data,” a demographer with the bureau’s population division, Gregory Harper, said. In the meantime, “we have to have a methodology that we can use for all counties,” he said.

The city successfully challenged the bureau’s estimates last year, resulting in a bump of more than 64,000 people in the revised figures. It plans to do so again this year, the director of the population division at the Department of City Planning, Joseph Salvo, said. He said the city believes its population is about 8.2 million people, which would mark an increase of less than 1% from last year’s revised estimates.

“We are in growth mode,” Mr. Salvo said. “We are planning for growth.”

Mayor Bloomberg has made preparing for significant population growth a major theme of his second term. The city expects the population to reach 9 million by 2020, and Mr. Bloomberg said in his State of the City address in January that he plans to lay out a strategic plan this spring to cope with the anticipated boom.

An economist and the co-founder of Urbanomics, Regina Armstrong, said she expects the city’s population to continue to grow. The demographics of that growth also puts the city in a solid economic position, she said. While the rates of increase in places like Phoenix and Scottsdale, Ariz., dwarf those of New York, they draw many retirees who do not contribute to the labor force. New York, on the other hand, “is attracting a lot of younger people, which keeps the city vibrant,” Ms. Armstrong said.

Analysts agreed that the city’s high cost of living, heightened by rising real estate prices, was a top challenge for maintaining population growth in the future. “Even though New York has done a great job with crime, and it’s more pleasant, it’s just too expensive,” Mr. Kotkin said. “There are a group of people who are locked out of the market permanently.”

Among the five boroughs, Staten Island (Richmond County) experienced the greatest increase since 2000, at 4.7%. Manhattan (New York County) grew by 3.6%. The Bronx, Brooklyn (Kings County), and Queens all showed increases of less than 2%.