As a senior planner with the Maryland-National Capital Park and Planning Commission, Philip Taylor normally combs statistics for clues about Prince George's County's future.

With the approach of the 50th anniversary of the Supreme Court's ruling against racial segregation in public schools, however, Taylor turned to census records to see how the landmark decision helped shape the county's present.

Taylor's findings, which he recently presented at a conference of the American Planning Association, provide stark illustration of the extent to which county leaders' response to the court's ruling in Brown v. Board of Education triggered two dramatic shifts in the county's makeup. The first caused Prince George's to become nearly all-white; the second, even more abrupt, changed it to majority-black.

Taylor cautions that Brown was not the only reason for the transformation, and notes that population statistics do not directly address residents' motivations for moving in or out of the county.

Still, he said, "you can look at the trends and draw some very interesting conclusions."

The story actually begins in the days of slavery, when blacks at times made up as much as 69 percent of the population of Prince George's.

After the Civil War, the white minority still in control of the county was loath to give up its power. In 1864, Prince George's voters overwhelmingly rejected a new state constitution that abolished slavery. It was narrowly approved anyway because of votes in others parts of the state.It is possible that the atmosphere of hostility toward newly freed blacks in the county prompted many to leave Prince George's. Whatever the reason, the county's black population declined by nearly one-third in the decade following the Civil War even as whites continued to move into the county. In the decades that followed, the trend continued, with the county's white population growing steadily as its black population declined, was stagnant or grew only slightly. The decade from 1940 to 1950 saw the biggest population boom, bringing the county's total population to a record 194,182. Much of the increase was due to the explosion in government jobs created by World War II. Workers from across the nation flocked to the Washington area. Like many surrounding rural counties, Prince George's was quickly built up with bedroom communities to house the influx.

Thousands of black workers also began moving to the county during this period, increasing their numbers by about 40 percent. They still faced substantial discrimination from home sellers and mortgage programs. Possibly for this reason, the number of black residents relocating to Prince George's during the 1940s was dwarfed by that of whites, whose presence increased by about 175 percent.

Then, on May 17, 1954, the Supreme Court declared racial segregation of public schools unconstitutional.

In theory, the decision had profound implications in Maryland, where state law had mandated separate schools for black and white children since 1872. However, leaders in Prince George's were slow to enforce it, initially adopting a "freedom of choice" system under which children were automatically assigned to their original segregated school unless their parents requested a transfer.

The effect of the policy was to keep county schools overwhelmingly segregated even as schools in the District of Columbia were integrated. This, in turn, said Taylor, may explain why white residents continued to move to Prince George's in droves even after Brown, nearly doubling in number from 1950 to 1960.

"We do not have data that specifically say that this was the reason," Taylor said. "But with all these other nearby jurisdictions coming up with integration plans as Prince George's is known to be resisting, it created an atmosphere that attracted people who didn't want their children attending integrated schools."

The number of black residents also continued to grow during this period, increasing once more by nearly 40 percent. But with so many more white residents arriving in the county, by 1960 blacks accounted for only 9 percent of the population.

In the mid- to late 1960s Congress and the federal government finally began to move against many of the discriminatory housing practices that had hampered blacks moving to Prince George's working class and middle class neighborhoods. By the end of the decade their numbers in the county had nearly tripled, while the number of whites grew by about 74 percent, making Prince George's the nation's fastest-growing suburban county.

Then, in 1973, a group of black parents won a court order forcing Prince George's to integrate its schools through busing. The following year county and state officials imposed a moratorium on new housing construction in many areas of Prince George's on the grounds that the county's sewer system had been overwhelmed by the construction boom of the previous decades.

The sewer moratorium remained in effect in various parts of the county until 1978, forcing many county residents who wanted to use their rising income to move to bigger homes to leave the county.

The combined effect of the moratorium and the busing order on the county's white population was dramatic. From 1970 to 1980, the white population declined by nearly a third, even as the black population continued to rise, this time by 170 percent. The net effect was to keep the county's population nearly stagnant, making Prince George's one of the nation's slowest growing suburban counties.

By 1980, blacks made up nearly 40 percent of the county's population. The trend has continued ever since, with the black population reaching 63 percent in 2000, a level not seen since the time of slavery.

There is, however, a crucial difference: This time, many of the blacks moving in were more affluent and educated than the white residents they were replacing.