The Rev. Harry B. Dalzell, 54, rector of Ascension Chapel Episcopal Church in Gaithersburg and former president of the Washington Episcopal Clergy Association, died Thursday at his vacation home in Cambridge, Md., after a heart attack.

He was president of the Diocesean Clergy Association from 1962 to 1965, and convenor of Convocation 9 (Montgomery County). He served as an officer of the Montgomery County Interfaith Clergy Association in 1967.

Mr. Dalzell was named vicar of Ascension Church in 1957, and became rector when the church was elevated to a parish in 1965. Before that, he had been rector of Christ Episcopal Church in Chaptico, Md., and rector of St. Bartholomew's Episcopal Church in Laytonsville.

He had been president of both the Gaithersburg Pastoral Counseling Center and Gaithersburg Help and was a founder and director of the city's Joint Venture Home.

Mr. Dalzell was a native of Mount Rainier. He attended Langley Junior High School and the old Central High School before earning a bachelor's degree at George Washington University. He was a 1952 graduate of the Virginia Theological Seminary.

He served in the Army during World War II.

He is survived by his wife, Anne, of the home in Derwood, Md.: three daughters, Janet Smith, of Columbia, S.C., Martha, of Orlando, Fla. and Mary, of Adelphi; his mother, Margarete Dalzell, of Satellite Beach, Fla., and a sister, Margarete Braxton, of Gainesville, Fla.

The family suggests that expressions of sympathy be in the form of contributions to the building fund of Ascension Chapel Episcopal Church.