A construction firm building part of the controversial extension of Interstate Rte. 66 in Arlington County has been the target of persistent and unusual vandalism.

Paint-filled balloons have been thrown on machinery, tar has been spread on tractor seats and windshields, door handles have been removed and trash has been stuffed in exhaust pipes and air intakes, according to Charles Scott, project manager for Expressway Constructors, Inc. Once, Scott said, the chains and binders holding down a load of logs on a flatbed truck were removed and taken.

"We think adults are involved, although we have no evidence," according to Lt. Roxy V. Crack, Arlington police spokesman.

The extension of I66 has been bitterly opposed by Arlington residents, who said the expressway would destroy neighborhoods and the local environment. A protracted court fight delayed construction for years. After being upheld by a U.S. appellate decision, the Virginia Department of Highways and Transportation let contracts on the project late last year and work began.

Scott said the vandalism begin in the first part of February. As recently as Tuesday night tar was spread on tractor controls windows - the second time that has happened, according to Scott.

Expressway Constructors is the prime contractor on the one-mile, $19.2 million section of I-66 being build between Lee Highway and Utah Street. An official of John Driggs Co., which is building another section in Arlington - between Sycamore Street and the county line - said that firm has not experienced such vandalism.

"We maybe have a little bit more security," said George Lang, project manager for Driggs. "Also, we keep most of our equipment in an enclosure, and it's lit with floodlights. And a number of our people tour the site at night."

Scott said Expressway has put a security guard on its site since the wave of vandalism began, but both he and police say the construction area is too extensive to patrol effectively.

Crack said the police are considering a stakeout "to try to catch someone in the act."

The vandalism has been mostly a nuisance, Scott said, and has not resulted in any significant delays in construction. He said Expressway hopes to finish its section by the spring 1980 deadline. Paving should begin in the fall, he said.

Scott said that, like the police, he suspects adults are at least partly responsible.Both Scott and police said the vandalism probably involved adults because it was so ingenious.

Scott said Expressway worked on two other major road projects in the area - the widening of the Capital Beltway and repairing of Interstate Rte. 95 - but the vandalism at those sites was isolated and consisted mostly of broken windows. The I-66 vandalism, he said, has been occurring "two or three times a week."