A four-alarm blaze that occupied as many as 150 Montgomery County firefighters early yesterday gutted half of a 67-year-old former grain mill on Gaithersburg's downtown business strip, leaving an automobile glass company in search of a new home.

The fire broke out shortly before 12:30 a.m. at Bowman's Mill on East Diamond Avenue and took more than two hours to bring under control, officials said.

A spokesman attributed the fire to arson, but would not comment further on investigators' findings. He said it caused about $450,000 worth of damage and required more personnel and equipment than any blaze in the county since a mill fire in Germantown in 1973.

There were no injuries, Capt. Raymond Mulhall of the Department of Fire/Rescue Services said.

Department spokesman Michael Hall said it was the third arson fire at the mill. The first one in 1964 gutted half the building and another in 1985 damaged an office, he said. Earlier, there was an accidental fire in 1948.

The three-story mill was constructed in 1920 of heavy timber and cinder blocks, which made it difficult for firefighters to hack their way inside, Mulhall said. Inside, he said, they had trouble reaching several partially closed-off areas that were ablaze.

The time-consuming job of dousing the burning heavy timber required an unusually large amount of water, Mulhall said, and empty grain silos connected to the front of the mill slowed the movement of aerial ladders. The four alarms, sounded between 12:30 and 12:47 a.m., brought fire trucks from 13 stations as far away as Chevy Chase.

Hall said the blaze broke out on the east side of the mill and destroyed space occupied by the Magic automobile glass company. A three-story concrete firewall helped limit damage to the adjoining J.W. Cullop Inc., a heating and air conditioning shop on the west side of the mill.

Fearing that the rumble of railroad trains passing immediately behind the mill might cause the east side of the building to collapse while investigators poked through the wreckage, Hall said, the department asked the CSX Corp. to slow its trains to 10 miles per hour in the area. He said they normally travel 50 to 60 miles per hour.

John Dixon, Gaithersburg's building code enforcement officer, said yesterday that he will order the east side of the building torn down. The Cullop company can continue to do business on its west side, he said.

The Magic glass company, opened in August, employs three installers and a secretary, all of whom will be put to work at Magic's three other shops in southern Maryland, said manager Darrell Hibjan.