in 1958 casselman was elected as grenville-dundas' mp for the final time, and was appointed deputy chairman of the house of commons committee by then prime minister john diefenbaker. the 1958 federal election won by arza clair casselman would be the last election he would ever run for, as he died on may 11, 1958 just months after the march 1958 election was held. casselman was 67 years old. later in the year, his wife jean succeeded him as mp for grenville-dundas.