As hyperparasitoids may have a considerable influence on the control of herbivorous arthropods, analyzing the host-mediated impact of Bacillus thuringiensis-plants ("Bt-plants") on hyperparasitoids is of interest. Laboratory-scale experiments were conducted in order to assess the potential effect of Bt-corn leaf material on the facultative hyperparasitoid Tetrastichus howardi (Olliff) (Hymenoptera: Eulophidae), mediated through the herbivore Chilo partellus Swinhoe (Lepidoptera: Crambidae) and its primary parasitoid Cotesia flavipes (Cameron) (Hymenoptera: Braconidae). In the Bt-group, significantly less C. flavipes larvae per host spun cocoons and pupated than in the control, and the mean fresh weight of a single C. flavipes cocoon was significantly reduced compared to the control. All C. flavipes cocoons of one host formed cocoon clusters. T. howardi females of the Bt-group parasitized significantly less cocoon clusters than in the control. Moreover, significantly fewer C. flavipes cocoons per cocoon cluster were successfully parasitized as compared to the control. As a consequence, T. howardi females of the control had more offspring than in the Bt-group. Adult female T howardi offspring of the Bt-group weighed significantly less than in the control, but there was no significant weight difference between males of both groups. Our results suggest that transgenic insect-resistant plants could affect hyperparasitoids indirectly. However, it remains to be determined whether facultative hyperparasitoids prefer to develop as primary or secondary parasitoids under field conditions. Biometrics; Reproduction; Life cycle and development; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Diseases and disorders; Microbial diseases; Ecology; Trophic structure; Abiotic factors; Chemical factors Chilo partellus (Pyralidae); Hymenopteran parasites; Cotesia flavipes; Mediation of effects of bacterial insecticide-treated plants on hymenopteran hyperparasite; Hyperparasitism; Bacterial diseases; Bacillus thuringiensis; Food webs; Fertilizers and pesticides Cotesia flavipes (Braconidae); Hymenopteran parasites; Tetrastichus howardi; Host-mediated effects of bacterial insecticide-treated plants; Lepidopteran hosts; Chilo partellus; Host-mediated effects of bacterial insecticide-treated plants on hymenopteran hyperparasite; Food webs; Fertilizers and pesticides Tetrastichus howardi (Eulophidae); Weight; Reproduction; Development; Hymenopteran hosts; Cotesia flavipes; Host-mediated effects of bacterial insecticide-treated plants; Hyperparasitism; Food webs; Fertilizers and pesticides none