Ben T. Plymale, 55, a vice president of the Boeing Aerospace Co. and a former Pentagon official who was deputy head of the Reagan transition team at the Defense Department, died Aug. 8 in a hospital in Rivers Inlet, British Columbia, after a heart attack.

A resident of Seattle, Mr. Plymale was on a fishing trip in Canada when he was stricken.

He was deputy director for research and engineering for strategic and space systems at the Defense Department from 1968 to 1972. In addition to serving as deputy team leader of transition at the Pentagon, following Reagan's election as president in November 1980, he was also cochairman of Reagan's Defense Budget Committee.

Mr. Plymale joined the Boeing Co. in 1950 and became an authority in strategic defense systems. He was one of the Boeing managers who developed the Minuteman missile program. He was the author of technical works, dealing with gyro-dynamics and navigation systems of missiles, many of which are still used in university instruction.

After leaving the Pentagon in 1972, he returned to Boeing as space systems division vice president. From 1974 to 1979, he was vice president for business development, and was Boeing's vice president for advanced missile programs at the time of his death.

He was reared in Oregon and earned a bachelor's degree in physics and mathematics at the University of Portland. He served in the Navy during World War II.

Survivors include his wife, Susan, a sister, and 10 children, all of Seattle.

The family suggests that expressions of symapthy be in the form of contributions to the experimental cancer program at the Northwest Oncology Foundation, 901 Boren Ave., Seattle, 98104.