Field research was conducted to describe and characterize spring emergence and seasonal flight activity of the cabbage maggot, Delia radicum L. (Diptera: Anthomyiidae), in relationship to degree-day accumulations. Turnip and rutabaga fields were monitored in the northern Willamette Valley in western Oregon from 2001 through 2004. Spring emergence from overwintering puparia was monitored using emergence cages. A bimodal spring emergence pattern was observed, with ≈70% of the spring population emerging in an early peak in late March, 2 mo before a later peak near the end of May. The mean degree-day accumulations at 10, 50, and 95% of spring emergence using a lower and upper developmental threshold of 4.3 and 30°C beginning 1 January had corresponding degree-day values of 200 ± 50.2 (8 March), 330 ± 22.2 (4 April), and 762 ± 60.1 (28 May), respectively. Seasonal flight activity was monitored using yellow water traps. Spring flight patterns mirrored the bimodal emergence patterns but with a delay of 3 d to 5 wk between emergence and detection of flies in the water traps. The mean degree-day accumulations recorded from the beginning to the end of spring flight had corresponding degree-day values of 303 ± 61.5 (31 March) to 839 ± 51.9 (4 June). Fly activity was lower over the summer from the beginning of June until the end of August (2,138 ± 82.3 DD). A fall flush of activity was observed each year beginning in late August to early September and extending through October (2,860 ± 170.6 DD).