Cone screen traps baited with cruciferous material were used in conjunction with thermal-unit accumulations to distinguish generations of Hylemya brassicae (Bouché), adults in the field. The emergence pattern from overwintering puparia was reflected in succeeding generations. In 5 years, the air thermal-unit accumulations (day-degrees) for the spring emergence of flies varied from 290 to 365 (with a developmental zero of 43°F) depending on weather and soil variations. Later generations required a mean accumulation of 1176 thermal units. Using this information, it is estimated that predictions of cabbage maggot adult emergences could be made as an aid to the timing of foliar sprays.