Female Encarsia formosa were enclosed in artificial arenas containing twenty-five Trialeurodes vaporariorum scales on a Cellophane substrate. Either the substrate or the scales were previously treated with chemicals. The number of oviposition wound marks on the scales was used as a measure of effect on parasite behaviour. The insecticides, tetradifon and γ-BHC, had their greatest effect when applied to scales. The fungicides, benomyl and dichlofluanid, were more active on the Cellophane. Pirimicarb applied to the scales decreased oviposition, but greatly increased it when applied to the substrate. It is concluded that even sublethal amounts of pesticides may seriously impair the effectiveness of this parasite in commercial biological control.