ATLANTA, April 16 — The rush to the open spaces of suburbia that transformed the United States for the last 50 years began to slow in the 1990's, an analysis of the latest census figures shows. While some suburban outposts established themselves outside the orbit of big metropolitan areas, many older suburbs neared their physical limits of outward growth and matured into cities.

The suburban landscape as it used to be known — a collection of treehouse backyards just outside towns but never far from woods or countryside — became increasingly scarce in the 1990's. The traditional movement outward from central cities became severely constrained by a variety of forces, like physical and geographical barriers, oppressive commuting distances and the air and water pollution caused by development. In response, as the census analysis and other demography studies show, suburbs took one of two contrasting regional paths that population experts say are redefining the nature of suburban life.

In the West, where land and resources are simply not available for growth, former suburbs filled in and reached urban-level densities. In the South the growth has slowed as well, though not as sharply. With few natural barriers to growth, many suburbs simply detached themselves from an old-fashioned central anchor and became, in effect, suburbs without urbs, free-floating patches of population density tethered loosely to interstate highways.

"A great many things were happening in the 1990's," said Bruce Katz, director of the Center on Urban and Metropolitan Policy at the Brookings Institution in Washington. "The first ring of suburbs were densifying and becoming more compact. The central cities started to become stronger. And much of the suburban growth leapfrogged into the far fringes, spreading out over a much larger landmass."

Demographers who have studied the phenomenon say the postwar concept of a suburb as an urban satellite was finally put to rest with the population shifts of the 90's.

"The geographical definition of suburb is outdated and has to be changed," said Leon Bouvier, a demographer at Old Dominion University in Norfolk, Va. "It's starting to fill in places we never expected, a kind of sprawl beyond sprawl as one suburb stretches out to meet another. There are those who may not like it, but if the country keeps on growing like it is, the people have to go somewhere."

The computer analysis of the latest census data, conducted by The New York Times, examined the population density in each of the country's 65,443 census tracts, defining urban, suburban and rural on the basis of how many people lived in a square mile, rather than the location of the tract. This makes it possible to observe suburban density levels in areas that are nowhere near a central city — the increasing phenomenon known as sprawl — or to detect a suburb thickening into a city.

But while sprawl may have dominated the headlines about growth in recent years, it now turns out to be a concept almost too simple to describe the population trends now under way.

Over all, the study showed, the nation's population became 15 percent more suburban in the 1990's — a significant slowdown compared with a growth of 21 percent in the 1980's. Because the South and West accounted for more than three-quarters of the country's 11 percent growth in population, the divergent paths of expansion were most distinct there.

In the West, the once-relentless growth of suburbs most clearly reached its limits. Where 3,600 square miles of rural land in the West became suburban in the 1980's, only 3,140 square miles became suburban in the 1990's, a 13 percent decline.

In the South, however, the amount of rural land that became suburban declined only slightly during the 1990's. In addition, the South's population became more than 19 percent more suburban, by far the greatest such increase in the country, even if it fell short of the 30 percent suburban increase during the 1980's.

"There's a big difference in how the two ends of the Sun Belt have been growing," said Robert E. Lang, director of urban and metropolitan research at the nonprofit Fannie Mae Foundation in Washington. "If you look at Atlanta and Phoenix from space, you'd see that Phoenix's development has been largely contiguous and has a high density boundary around it. It's constrained by aridity and the slope of the land. But Atlanta, with no such barriers, has been growing at a much lower density, with development jumping over big patches of empty land. The Southeastern cities are far less dense than the West."

Mr. Lang's own research has shown that 9 of the nation's 10 lowest- density metropolitan areas are in what he calls the "wet Sun Belt" of the South: Raleigh, N.C.; Nashville; Charlotte, N.C.; Greensboro, N.C.; Jacksonville, Fla.; Richmond, Va.; Louisville, Ky.; Oklahoma City and Memphis. By contrast, 8 of the 10 highest density metro areas are in the West: Las Vegas, Los Angeles, San Diego, San Francisco, Phoenix, Sacramento, Seattle and Portland, Ore.

Suburban density levels in the South began to appear in areas much farther from cities than ever before. Bands of suburbs have started to merge with each other along Southern transportation corridors, in some cases forming almost unbroken chains of medium-density areas hundreds of miles long — from Atlanta to Raleigh along Interstate 85, or from Washington to Norfolk.

Demographers say these converging bands of Southern suburbs will soon begin to rival in area, if not in density, the great metropolitan corridors of the Northeast and industrial Midwest. But these population clusters are not centered around great ports or waterways as in previous eras; instead, they are suburbs of interstate highways.

"These are fully suburban megalopolises, and we've never seen that before," Mr. Lang said. "The Southeast is evolving into this huge countrified city across a vast space."

The Northeast and Midwest fall somewhere in between. The rate of suburban growth fell in the Northeast, while it rose in the Midwest. But both regions experienced significant population increases in the cities, compared with the previous decade. Urban growth in the Northeast was 3.3 percent in the 90's compared with 0.9 percent in the 80's; in the Midwest the urban population grew by 1 percent, whereas it actually fell by 3 percent during the 80's.

Over all, the country's population is now 12 percent more urban than it was in 1990. Demographers say this is the result of several trends: not only are suburbs reaching urban density, but families and immigrants are moving back to central cities. Several major cities that experienced declines in previous decades — Chicago, Atlanta, Denver, Boston — saw growth in the last 10 years. New York City pushed past eight million people, expanding faster than its inner suburbs (though the suburbs at the fringes of the metro area grew even faster than the city).

"I think you could argue that the cities are beginning to turn it around, for the first time in a long time," said Kenneth T. Jackson, author of "Crabgrass Frontier: The Suburbanization of the United States," and a history professor at Columbia University. "If the 20th century was the suburban century, I think in the 21st we will start to see a new appeal of urban density."

Professor Jackson attributed the shift to the decrease in urban crime across the country, and the preference of young professionals and immigrants for the close interactions of city life.

Some of these trends are a direct outgrowth of each region's history, shaped over the decades by geography and immigration patterns. Michael Ratcliffe, a geographer at the Census Bureau who works on urban issues, said the South's unconstrained spread stemmed from its agrarian past, when its countryside filled up with small settlements at a much greater rate than in the Midwest or West. Those little crossroads are now growing out to meet each other, Mr. Ratcliffe said, while the urban concentrations elsewhere become more concentrated.

If the urban renaissance hints at the beginning of a new movement, one older trend that continued unabated is the rapid development of once-rural land. More than 17,000 square miles of land that was rural in 1990 reached suburban or urban densities by 2000, the analysis found, an amount more than twice the size of New Jersey.

A similar amount of land was developed in the 1980's. Fifty acres of forest are lost every day in the Atlanta area alone, and Calvin L. Beale, senior demographer with the United States Department of Agriculture, said about two million acres of farmland are being developed every year.

More than half of the rural loss is in the South, and the effect can easily be seen on a map. Enormous areas that once contained the region's agrarian hamlets have grown to full suburban densities, even though they are not really suburbs of any city in the traditional sense.

In Habersham County, Ga., for instance, nearly a two-hour drive from both Atlanta and Greenville, S.C., suburban densities began to appear for the first time in the 90's in a county that was considered entirely rural a decade earlier. The county grew by 7,800 people over the decade to 36,000 people, a 28 percent increase, and many of those newcomers were Hispanic workers at the county's poultry plants.

But Harry Carter, the city manager of Cornelia, the county's largest city, said many of the new residents had escaped the metro areas of the I- 85 corridor and had recreated a suburb in his small town.

"They're here now, and the Wal- Mart and the Kmart and the four- lane highway have followed them," Mr. Carter said. "It used to be you couldn't hear much traffic around here, but now it's a constant hiss. We're going to have to think about upgrading our old sewage treatment plant. I hope this doesn't mean we're going to become Charlotte one day."