For decades, Prince George's County has been, if not the ugly duckling of the Washington area, at least its latest bloomer.

It was literally on the wrong side of the tracks. The whites who lived there were stereotyped as rednecks. The police force had a reputation for brutality. The schools were a cauldron of racial controversy. The income levels of its residents did not match those in Fairfax County. The housing did not have the cachet of Montgomery County's.

Even blacks denigrated the county's supermall, Landover, calling it "Black Flint," a snide comparison with the high-fashion, high-priced White Flint Mall in Montgomery.

Today, however, the last laugh is being enjoyed by the 676,000 people who live in Prince George's. The county has been discovered by developers, builders and retailers, and, of all the middle-class suburban counties in the United States today, Prince George's is among the most racially heterogeneous and integrated -- arguably the embodiment of the American Dream.

This puts the county -- which is as populous as San Francisco and is about 46 percent black -- on the forefront of the emergence into the suburbs of a new black middle class without precedent in size and accomplishment in American history. Also, it makes Prince George's, with its 30-year history of black suburbanization, one of the nation's foremost laboratories of cultural change -- for whites as well as blacks -- as ethnic diversity comes to America's bedroom communities.

"There's no question in my mind. The white outflight has been turned around. We're seeing an absolute increase in the white population in this county for the first time since the '60s," Prince George's County Executive Parris Glendening said.

"Prince George's County is going to be an incubator for national leadership," said Clayton J. Powell Jr., who was born in the county 30 years ago, is black, and who has returned, after graduating from Harvard Law School, because of the opportunities he sees there.

"You have integrated demographics and a highly educated populace in the shadow of the nation's capital. This is going to be a good breeding ground of a new style of leadership that's going to carry the country into the 21st century."

"In my wildest, wildest dreams, I didn't think the value of ground would increase as it did in Bowie," said Mark Vogel, a 39-year-old white who was brought up in Prince George's and who is now a developer. "I paid $15,000 an acre for it three years ago. Commercial ground there now goes for $320,000 an acre. I own over 2,000 acres in the county. Maybe closer to 3,000. You don't have to sell Prince George's County anymore."

"The driving force of the success of Prince George's County is class," said Glendening, who is widely credited with addressing the negative perceptions of the county, including the accurate ones that some of its neighborhoods are little more than extensions into the suburbs of the District of Columbia's most poor and drug-beset areas.

"The thing I find revealing is that when I go out into the communities and speak with blacks, the aspirations are exactly the same as you would expect to find in an upper-middle-class white family. There is little discussion of social services. We're not talking central-city race politics. We're talking 'Protect my land values.' 'I want those schools to be the best possible so my kids can do as well as I did.' The quality of life in the neighborhood. Traffic."

"Prince George's," pointed out Albert Turner, "might have grown slower if it were not for the black middle class."

Turner, who is white, is at 70 the grand old man of development in Prince George's. In the 1960s, he and what grew to be more than 200,000 quiet pioneers -- members of the new suburban black middle class -- saw the potential for the county. The six Ferraris in his garage are a tribute to his foresight.

Among the projects Turner developed was Kettering, a subdivision of three- and four-bedroom, two-bath houses that, in the late 1960s, was the first in the county to attract large numbers of black buyers with incomes that were well above average.

"Development is pretty much a numbers game," said Turner. "Commercial development follows income levels. If you have a couple whose income is in the $80,000 range, they need groceries, automobiles and jobs, just like anyone else. There's nothing in their numbers that says whether they're black or white."

Arie Stouten, who is white, moved to racially mixed Cheverly not long ago from an almost exclusively white beach community in Anne Arundel County. "I considered it," Stouten acknowledged, referring to the question of race. "But it wasn't overwhelming enough to keep me from coming in. It was just there as a factor, like the adequacy of the roads. The biggest thing was it was the most house for the money. We have two kids. I looked at some of the demographic reports and picked an area where the housing value was holding steady. That was much more important than any racial factors."

"Have you ever seen the movie 'To Kill a Mockingbird?' " asked attorney Powell, whose parents moved to the North Brentwood section of Prince George's -- out Rhode Island Avenue between the District and Hyattsville -- in the first wave of black suburban migration of the late 1950s. "That was Prince George's County when I grew up. Small town, very redneck, kind of slow. Nobody wanted to give me a chance . . . . It was that experience that really compelled me to study the law."

But it was not Prince George's County's past that brought Powell back. It is the extraordinary changes that he perceives to be its future. "So many well-educated blacks from across the country," he mused. "This gives us tremendous opportunities. I see Prince George's working itself into a cauldron of leadership nationwide. Prince George's is becoming a switching point for that black power elite, giving us a first crack at leadership right here in our community. More so than in the District because there is no question of home rule. Once we get control, it will be ours."

Indeed, with its minority procurement legislation, divestiture from South Africa and appointments of blacks to key positions, the traditional white political hierarchy in the county, including Glendening, has acknowledged the pressures being put on it by this emergent black political force.

But for all the grand implications of the demographics of the county, the basic reasons why the new black middle class moved to Prince George's were the same as for those Americans who moved to the suburbs.

"We were in our late twenties, early thirties, looking for a nice, decent place for our families," recalled James Fletcher of the year 1962. "We were living in an apartment at the time. Renting. We didn't have as much money as those who went to the Gold Coast" -- the home of the old black elite on upper 16th Street in Northwest Washington.

Fletcher then was a GS 5 federal employe at the National Institutes of Health. He is now mayor of the predominantly black suburban town of Glenarden, having retired recently as a GS 14 computer systems analyst for the Department of Housing and Urban Development.

"We could have remained in the District, but we thought in Prince George's we had better value for our dollar. There was more room. The green area where you walk all around your house. Prince George's was also viewed as a safer place than the District. Maybe the schools, too."

This bench mark county, which stretches from the Potomac River to the Patuxent River with almost 500 gently rolling square miles, touches the entire eastern half of the Washington area and is more populous than seven states.

It is home not just to tobacco barns, but also to the University of Maryland, one of the 10 largest in the United States; the world's largest space research facility, the Goddard Space Flight Center; the world's largest agricultural research facility, the U.S. Department of Agriculture's 7,200 acres in Beltsville; the National Meteorological Center, whose supercomputers are the basis for the United States' weather forecasts; the U.S. Bureau of the Census headquarters in Suitland, and Andrews Air Force Base, the home of Air Force One.

Prince George's is home for the headquarters of such major local businesses as Giant Food and Hechinger's, as well as units of more than 200 high-technology companies, including General Electric, ITT, Martin Marietta, Sony, Unisys, Control Data and Digital Equipment.Large, Quality Development

The county's urbanization as measured by the construction of office space is growing rapidly, and the proposals for development are increasingly of high quality and large scale. It is expected to be a decade or more before any of these places achieves the critical mass of 5 million square feet of office space that is one of the definitions of an emerging city such as Silver Spring. Nonetheless, at least four prototype emerging cities seem to be taking shape. And they are well distributed around the county. {See map, Page A15.}The north end of the I-95/Rte. 1 corridor near Laurel. One of the developments there for which ground was broken this fall is Konterra, a 2,000-acre project with plans for 9 million square feet of office space, as well as hotels, research and development space, retail units and 7,000 homes. The Capital Beltway arc anchored by the New Carrollton station on Metro's Orange Line and the planned Greenbelt station at the end of the Green Line, including College Park and Landover. Capital Office Park, for example, is a 90-acre development that already has five large office buildings at the Beltway and Kenilworth Avenue and is expected to provide 1.2 million square feet of office space upon completion. The Bowie area near the interchange of Rtes. 50 and 301. Bowie New Town Center, on more than 200 acres, has 110,000 square feet of office space under construction. A substantial mix of office, retail and residential space is planned. The south part of the county around the Beltway and the Potomac. The PortAmerica project on 442 riverfront acres is planned to include an octagonal, 22-story hilltop tower and six 10-story office buildings with 2 million square feet of space, housing, retail space and a 500-slip marina. Ground breaking for the sales office at the site was held last month.

Prince George's is strategically located between the attractions of the District of Columbia and the Chesapeake Bay. With major north-south and east-west highways in place, it has the most complete road network of the huge urbanized counties in the Washington area. Rte. 50 is being upgraded to interstate status and will be rechristened I-68. Gov. William Donald Schaefer is promoting the Rte. 301 corridor as the Maryland portion of a proposed outer beltway.

Though unemployment is at a rock bottom 3 percent and the county's household income is in the top 2.5 percent of the United States, not all is rosy.

Many garden apartment complexes hastily constructed in the '60s have become centers of welfare dependency and drug dealing, making them little more than extensions into the suburbs of the District's most crime-ridden and run-down neighborhoods. Bedford Station Apartments on University Boulevard in Langley Park and Glenarden Apartments on Hamlin Street in Glenarden are among them.

Last week, the county marked a record total of homicides, surpassing the 86 slayings of 1982 with more than a month left in this year. As many as a third of those slayings are drug-related, county police officials say.

Some of this assessment is debatable, of course. Sugar Ray Leonard is almost invariably described in the media as having "escaped" from his childhood home of Palmer Park.

Palmer Park is a world of duplexes and triplexes, surrounded by low chain-link fences. Some of the homes need paint, as do the cars parked in front of them. However, many homes have been renovated recently, and all compare favorably with most white ethnic working-class housing stock in the Northeast part of the nation.

Alex Williams, the county's first black state's attorney, cautioned, "Some people are wondering -- have we reached a plateau? The question is, are we going to continue that kind of progress?"

Williams and other black leaders express a genuine sense of embattlement, seeing threats from without in the Reagan administration's reining in of social programs and opposition to affirmative action programs. But they also are worried about the threat from within.

"Take our young black males, their lack of role models, drugs," Williams said. He questions whether educational advances will continue, and he worries about the breakdown of the strong family unit, noting that "the mother always used to be home holding the family together. Now educated women are competing with males. This raises new problems."

As far as the Prince George's County's police are concerned, in a recent University of Maryland poll, most respondents said they thought that the police were a credit to the community. But more than one-third think that police use excessive force.

And when it comes to retail development, one commercial real estate broker notes that he does not see fancy shops such as Williams Sonoma, Eddie Bauer, Britches and Laura Ashley breaking down his door to get into Landover Mall.

Nonetheless, the prevailing view in Prince George's is positive. "There is a significant antigrowth movement in other jurisdictions. But here {growth} is seen as a tremendous avenue of opportunity," Glendening said.

Test scores have risen dramatically since its popular school superintendent, John A. Murphy, took over in 1984. The goal is for the system to be in the top quarter of the nation by 1990. Its academically challenging magnet school program is so attractive that, despite the addition of 2,500 slots this academic year, more than 2,000 students were left on the waiting list. Student discipline has improved, measured by a sharp decline in vandalism and disorderly conduct reports.

The turnaround of the schools is thought to be a large part of the reason for the county's major demographic shift -- the stabilization of its extremely mixed racial and ethnic composition, which now includes Asians and Hispanics.A Stable Racial Mix

After the 1980 census, demographers were predicting that continued white flight would result in Prince George's County becoming majority black by 1985. But by 1985, planners had begun to push back the expected date of that tipping point -- one that would signal a significantly less pluralistic Prince George's County -- to 2000, even 2010.

Now the Grier Partnership, the area's leading private demographers, says it thinks that the percentage of blacks in Prince George's County may never exceed its current level, which it estimates to be 46 percent.

If the 1990 census shows this to be true, two factors will have contributed, according to the Griers; Philip Taylor, Prince George's County's demographer, and others.

Not only will white flight have ended, it also will have been reversed, signaling an increased appreciation on the part of whites that the county has much of the area's most affordable housing close to downtown Washington.

Further, black migration into Prince George's will be shown to have slowed. This correlates not only with an increased black affluence that makes a house in Montgomery or Fairfax increasingly attainable, but also with new levels of comfort on the part of the black middle class with the idea of living in these predominantly white suburbs.

It is all economics, Taylor thinks. People care about the value of their house, and the quality of the schools, more than they do the color of their neighbors' skin, he says.

"In talking to people, I was made aware that Prince George's County had a large contingent of black people," said Kirk Howard, who recently moved into a neighborhood of three-level town houses in south Bowie from a white neighborhood of Anne Arundel County. "But it didn't matter that much to me. Around here, the people are the same class as you are. In order to be able to afford something like that, you have to be able to make good money. They have backgrounds similar to mine . . . . This is a very clean community."

"Property values falling when blacks move into the neighborhood is disappearing," said University of Maryland professor Bart Landry, author of the new book "The New Black Middle Class." "It's a qualitatively different atmosphere than when the scare was that when a black moved in, the end is near. People can think a little more rationally about their choices, weighing the choice between moving to Montgomery County versus living next to some middle-class blacks."

Developer Vogel agreed. "Five years ago, you couldn't get builders in Northern Virginia and Montgomery County to return your phone calls. It was company policy. Now every builder calls me. 'Do you have lots in Prince George's?' The market is unbelievable.

"You put two and two together."