Mary M. Dallas, 32, who was active in volunteer work and church groups in Prince George's County, died Saturday in the Doctor's Hospital of Prince George's County, where she was being treated for polymyositis, a muscle disease.

Mrs. Dallas had been a volunteer reading instructor and substitute teacher at Seabrook Elementary School in Seabrook for the past three years. t

She was secretary of the Lanham Boys Club in 1978 and coached a group of 7-year-olds to a baseball championship for the club that same year. She did much of her coaching from a wheelchair.

Mrs. Dallas belonged to St. George's Episcopal Church in Glenn Dale and was a member of the church's Ecumenical Christian Women's Guild.

Mrs. Dallas was a native of Chicago, and lived in Charleston, W.Va., before moving to this area in 1965. She attended George Peabody College for Teachers in Nashville, Tenn., and was a graduate of the University of Maryland.

Her first marrigae ended in divorce.

Mrs. Dallas is survived by her husband, David, a son, Mark Fero, and a daughter, Shannon Fero, all of the home in Seabrood; her parents, Mr. and Mrs. James T. Maloney, and a sister, Susan, all of Virginia Beach, and another sister, Particia Calhoun, of Williamsburg.