Experiments under controlled laboratory conditions were performed in order to determine the effect of B.t.-maize leaf material on the parasitized herbivore C. partellus (Lepidoptera: Crambidae) and on its parasitoid C. flavipes (Hymenoptera: Braconidae). The relative consumption rate (RCR) and other nutritional indices of parasitized hosts exposed to Bacillus thuringiensis-maize leaf-material are strongly reduced compared to the control, but only during the first 4 days of the experiment. Moreover, the fresh weight of parasitoid cocoon silk is lower than in the control, as well as the fresh weight of parasitoid pupae and the dry weight of emerged parasitoid adults. Only in the B.t.-group, strong negative correlations were found between the amount of food intake by the host and the number of parasitoid cocoons containing pupae. Strong positive correlations were also found only in the B.t.-group between food intake and the parasitoid development time. We conclude that controlled experimental procedures as used in this study should be applied in future laboratory studies to come to more precise conclusions about host-mediated cause-effect-relationships between B.t.-plants and parasitoids. Animals and man; Control; Biological control; Nutrition; Diet; Life cycle and development; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Pollution Chilo partellus (Pyralidae); Control by; Cotesia flavipes (Hymenoptera); Food plants; Zea mays; Development; Hymenopteran parasites; Cotesia flavipes; Parasitoid performance, effects of Bt maize, bioassay Cotesia flavipes (Braconidae); Control of; Chilo partellus (Lepidoptera); Lepidopteran hosts; Chilo partellus; Parasitoid performance, effects of Bt maize, bioassay; Biological pollution; Bt maize none