The seasonal occurrence of adult carrot rust flies,Psila rosae(F.), in the Holland Marsh, near Bradford, Ontario, was determined from 1972 to 1980 with the aid of emergence cages and sticky yellow traps located in insecticide-free plots of carrots. Dates of first emergence of the overwintered generation (OWG) ranged from 16 May to 1 June, and those of the first generation (1G) from 22 July to 3 August. Indices based on mean cumulative degree-days (DD) to observed seasonal events over 4 to 7 years permitted prediction of first emergence or first trap catch within 1 to 1.8 days of observed dates. The indices (with mean error of prediction) were: first emergence of OWG adults, 236 DD (air) above 5°C after 1 April (1 day) or 220 DD above 3°C (soil, 5 cm) after 1 April (1.5 days); first trap catch of OWG adults, 258 DD (air) above 5°C after 1 March (1.5 days); first emergence of 1G adults, 971 DD above 3°C (soil) after date of first emergence of OWG adults (1.5 days) or 1,050 DD (air) between 5 and 20°C after 1 March (1.8 days); and first 1G trap catch, 1,150 DD above 5°C after 1 March (1.6 days).