Predawn rains dampened groundcover and helped choke off a large forest fire that was threatening woodlands in the southwest Virginia mountains yesterday, forestry officials said.

The moisture, along with efforts by more than 230 firefighters and a host of volunteers, brought Virginia's worst forest fire in 15 years under control, said John Graff, chief of fire management for the state Division of Forestry.

"We do have a line around the fire," said U.S. Forest Service spokesman Terry Seyden. "It is over 90 percent contained. Very little smoke has been observed anymore. The crews are doing the mop-up."

By late afternoon, some of the federal and volunteer firefighters who battled the 1,616-acre blaze in Botetourt and Alleghany counties, 60 miles north of Roanoke, were allowed to go home, Seyden said.

A firefighting helicopter remained on the scene, but he said there was no longer a need to drop fire-retarding chemicals on the blaze from World War II vintage airplanes.

The stubborn fire swept through remote privately owned lands in Botetourt and Alleghany counties and about 300 acres in the 700,000-acre Jefferson National Forest. Before it was brought under control, the fire crept over Rich Patch Mountain to a small section of the George Washington National Forest.

Seyden said cost of the firefighting effort was about $188,000 and would be shared by the federal and state governments.

So far this year, 1,064 fires have raged over 8,375 acres of private, state and federal woodlands, exceeding the number in the 1985 fire season, said Virginia forestry division spokesman Lou Southard.

Officials say the latest fire, which is also the worst of the 1986 season, pushed statewide damage totals from fire losses to more than $8 million. Estimates, including timber value and recreational use, are based on $1,000 for every lost acre. Fire officials do not know if the fire was accidentally or deliberately set.

Fishermen entering the woods for the first day of trout season today were also cautioned to remain on guard because forest lands remain a potential tinderbox.

"The word I want to get out is don't get overconfident," Seyden said. "We have had some light rains across the state and they may be more harmful than helpful because they give people a false sense of security."

Forecasters said most parts of the state haven't had a measurable rainfall in two weeks, and there's little chance of rain before Monday.

State forester James Garner has said he may ask Gov. Gerald L. Baliles to close the state's forest lands if fires increase with the season opening. Baliles, in Southwest Virginia for two days of transportation meetings, was scheduled to view the charred acreage from the air.