Three-quarters of Virginia's population growth in the 1980s has occurred in 18 localities concentrated in the eastern third of the state, according to the University of Virginia's Tayloe Murphy Institute.

"Population growth in the state is highly concentrated," said David W. Sheatsley, an institute research analyst who prepared an annual population report with Julia H. Martin, director of the institute's Demographic Studies Center.

The 18 localities with growth rates of 10 percent or more during the 1980s cover just 12 percent of the land area in Virginia, Sheatsley said. "There's pretty much a shift to metropolitan areas," he said. "It's primarily the rural localities that are losing population."

Fairfax County, with 687,800 residents in 1985, remains by far the largest and is one of the fastest growing of Virginia's 136 cities and counties, with a 15.5 percent growth rate since 1980.

Virginia Beach is the state's largest city with 319,400 residents, the report said. The resort city has grown by 21.8 percent since 1980, it said.

Manassas has added 4,000 residents since 1980 to give the Northern Virginia city a population of 19,500 and a 25.9 percent growth rate.