Previously, we examined the effect of insecticides on four species of pteromalid parasitoids and their host, the house fly, Musca domestica L. Generally, the parasitoids are more susceptible to insecticides than the flies. In this study we examined the effects of synergists that are known to block oxidative (piperonyl butoxide) (PBO) or hydrolytic (S,S,S -tributylphosphorotrithioate) (DEF) metabolism on the toxicity of five insecticides to M. Domestica and the pteromalid wasp, Muscidifurax raptor Girault & Sanders, to evaluate if metabolism was involved in the differential toxicity observed in the fly-parasitoid system. No consistent differences in PBO- or DEF-enhanced insecticide toxicity between species were seen. This suggests there is no general rule that can be made regarding increased levels of monooxygenase or hydrolase activities between host versus parasitoid species. Although the differential toxicity of certain insecticides to M. Raptor and M. Domestica is due in part to differential metabolism, it is clear that other processes must also be involved.