The cabbage maggot, Hylemya brassicae (Bouché), is one of the main factors limiting the production of high-quality rutabagas, Brassica napobrassica Mill., in the irrigated districts of southern Alberta. This crop is damaged late in its growing season; there is little damage to cruciferous crops in early spring except that occasionally early-transplanted cauliflowers are killed. This suggested the possibility of some change in the cropping practice if the flight and oviposition periods, recorded herein, were accurately known.