Growth in Fairfax and Montgomery counties, as motorists on I-66 and I-270 already know, surged again last year as the Washington area's population boomlet of the 1980s continued, according to new U.S. Census Bureau estimates.

Population growth also picked up in Prince George's County, after more than a decade of little change, while the area's core -- the District, Alexandria, and Arlington -- stabilized or grew slightly in contrast to major population losses in those jurisdictions during the 1970s.

The population of Fairfax County has reached 710,500, the highest in the area for the second year in a row -- up 22,700 from mid-1985 and 114,800 since the 1980 census. Most of the growth has been in western Fairfax along Rte. 28 in an arc from the Herndon-Reston area to Chantilly and Centreville.

Most of the growth in Montgomery -- which grew by 21,300 from mid-1985 to 665,200 -- has been in the Gaithersburg-Germantown area along I-270, and in Burtonsville along Rte. 29.

In all these areas, as in Fairfax, much of the new construction has been in relatively modest-priced town houses.

Prince George's was the area's second-largest jurisdiction with a population of 681,400. The Census estimate is up 5,800 in a year -- much less than either Fairfax or Montgomery but more than in any single year since the 1960s when Prince George's was the fastest-growing county in the area.

Meanwhile, rapid growth continued on the fringes of the metropolitan area -- Stafford, Loudoun and Prince William counties in Virginia; Charles and Howard counties in Maryland -- but the increases were less explosive than they had been during the previous decade.

"It's the suburbs that are becoming more urban," said John McClain, director of metropolitan development for the Metropolitan Washington Council of Governments. "There's been a very high growth rate in jobs in both Fairfax and Montgomery, and many of the peo- ple who get those jobs are in two-earner families. Both want to get to work without a real long commute. They're willing to take less of a yard in order to be more convenient for the other needs of the family, such as day care."

Overall, the population of the Washington metropolitan area -- an agglomeration of the District, 10 counties, and five suburban cities -- reached 3,563,000 in mid-1986, the Census Bureau said. Since 1980, the area has grown by 9.6 percent, up 312,100 in six years, compared with an increase of 210,614 or 6.9 percent during the 1970s.

Fairfax and Montgomery combined account for almost two-thirds of the area's 69,400 population increase last year. Since 1980, the population of Fairfax has risen by 19.3 percent and Montgomery by 14.9 percent.

Both the number of new residents and growth rate are much less than they were during the boom years of the 1950s and 1960s, but the Washington area is again growing faster than the nation as a whole. During the 1970s, the population growth rate in the Washington area was 40 percent slower than the average nationwide, the first time it had lagged behind the nation since 1930.

Since 1980, growth here has been about 50 percent faster than the national increase, which is about the same as it was during the 1970s.

McClain said the upturn in population growth followed an upturn in jobs, particularly in Fairfax and Montgomery counties. Fairfax has benefited significantly from the defense buildup undertaken by the Reagan administration.

Montgomery has a large number of defense contractors as well, McClain said, but its economy also has been stimulated by biomedical research tied to the National Institutes of Health in Bethesda.

"These counties have a substantial service-information sector tied to the federal government," McClain said, "and that's a very good position in which to be."

The town homes which are housing so many of two counties' new residents "are more affordable for the people who are getting the new jobs," McClain said. "The land prices have been pushed so high that developers are building town houses instead of single-family homes to get the return they want."

He noted that in the past year commercial construction has picked up in Prince George's County, where land prices are still relatively low. "It's about time for Prince George's to get its share of job growth and population increases," McClain said.

The District of Columbia ranked fourth in the area with 626,000 residents in mid-1986, the Census Bureau said. This is 3,000 more than the bureau's revised D.C. estimate for 1985, though unchanged from the provisional estimate it issued for the city a year ago.

The D.C. figures were released in late December along with estimates for the 50 states. Yesterday's Census Bureau report contained figures for the nation's 3,138 counties and independent cities. The estimates were based on several complex formulas, using birth and death statistics, and income tax, Medicare and immigration records.

According to the new 1986 estimates Alexandria had a population of 107,800 and Arlington 158,700. Both were up about 4 percent since the 1980 census, which showed that Alexandria's population had dropped by 7 percent and Arlington's by 12.4 percent during the previous decade. The District's population fell by 15.6 percent during the 1970s.

The new stability in these central areas apparently reflects job growth, some of it stimulated by the Metro subway system; substantial settlements of Asian and Hispanic immigrants, and less out-migration of families.

Among the outer suburbs, Prince William has reached 175,400, an increase of 21.2 percent since 1980; Charles has reached 89,000, a six-year gain of 22.3 percent; and Howard, just outside the official metropolitan area, 151,200, a gain of 27.6 percent in the same period.

Even though these fringe counties are still the fastest-growing in the area, the annual growth rate in all of them is less than it was in the 1970s. In most the actual number of new residents each year also is down from the previous decade, although Howard County reported an exceptionally large increase of 8,100 last year.