The effect of constant and sinusoidally cycling temperatures on the durations of combined egg and larval development and of pupal development of Liriomyza trifolii (Burgess) was studied. The observed and estimated durations accorded well using 147·5 day-degrees above 10 1°C for eggs and larvae and 138·7 day-degrees above 10·8°C for pupae. A few pupae survived for 76 days in outdoor cages during a winter in southern England. Eggs hatched at 6–16°C (cycling), but larvae did not complete their development under this regime. Adults emerged from pupae at 6–16°C, and a few deformed adults emerged after 65 days at 11·5°C (constant). Some pupal development was detectable at 8°C. Pupae exposed to 1·1°C for 20 days all died, but some adults emerged at 14–26°C from pupae that had been kept at 1·1°C for 15 days.