Aphidius colemani Viereck (Hymenoptera: Aphidiidae) showed a development time extended by 16% when reared on Myzus persicae Sulzer (Hemiptera: Aphididae) of different instars on artificial diet or on aphids on Brussels sprout plants. There was also a small but statistically significant 2% reduction in size (in hind tibia length) of adult females. Rearing on diet had no significant effect on the number of eggs females contained at emergence. In a second experiment, aphids from Brussels sprouts in instars 2~5 were compared with aphids from diet two days older to give comparably sized aphids from both substrates. Parasitisation by A. colemani gave the same direction of results, but showed that the effects in the first experiment were greater than predicted from the reduction in host size alone. However, in both experiments effects on parasitoids of diet-rearing the host were small, and culturing A. colemani on diet-reared aphids was successful. none