Retired Lt. Col. Robert L. Bierly, 71, who was assistant director and then director of the U.S. Army Field Band for 20 years, died Tuesday at Walter Reed Medical Center. He had cancer.

After his retirement from military service in 1966, he served for eight years as director of music at Virginia Military Institute in Lexington, where he resided at the time of his death.

While with the Army Field Band, which was based at Fort Meade, Col. Bierly resided in Silver Spring.

He was born in Renova, Pa., and grew up in Clearfield. He was graduated from Ithaca College in New York with a degree in instrumental music. His specialties were piano, organ and trombone.

Col. Bierly taught music in the public schools of Brockway, Pa., and Clarion, Pa., and then became director of vocal music in a Lynchburg, Va. high school in 1936, when he also joined the 246th Coast Artillery Band of the Virginia National Guard.

The band was one of the National Guard units to be called to active duty in 1940. Two years later, he completed a course at the Army Music School in Washington. Later, he reorganized and commanded the 88th Infantry Division Band and served with it in North Africa and Itally in World War II. He joined the Army Field Military Band at Fort Meade after the war.

Col. Bierly was a Mason and a member of the American Band Masters Association.

Survivors include his wife, Martha G., of Lexington; a son, Robert Drew Bierly of Rocky Mount, Va., and a stepbrother, John C. Rice in Northampton, Mass.

The family suggests that expressions of sympathy be in the form of contributions to the American Cancer Society in Lexington.