The ecological risks of transgenic plants, especially the influence of transgenic B.t.-maize on food webs have received increasing attention. Many field and laboratory studies have concentrated on bior tritrophic interactions involving transgenic B.t.-maize, herbivores, predators and primary parasitoids. So far, no study has been conducted to assess potential effects of B.t.-maize on secondary endoparasitoids, representing the 4th trophic level. The parasitized herbivore Chilo partellas (Lepidoptera: Crambidae) was fed with two different sub-lethal amounts of B.t.-maize leaf material, which ensured successful development of the gegarious parasitoid Cotesia flavipes (Hymenoptera: Braconidae) larvae inside the host. After eclosion from the host, the C.,flavipes larvae spin their cocoons and pupate, forming cocoon clusters. Parasitization parameters of the hyperparasitoid T. howardi attacking the cocoon clusters of C. flavipes were measured. The percentage of cocoon clusters parasitized by T. howardi was significantly reduced in cocoons that had developed in Bt-reared C. partellus larvae, compared to the control. Furthermore. the number of parasitized cocoons per cluster was significantly lower and the dry weight of emerged females of T. howardi was reduced in the B.t.-group. The results demonstrate, that B.t.-maize can have at least an indirect hostsize-mediated effect on the performance of a secondary parasitoid. Animals and man; Control; Biological control; Nutrition; Diet; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Pollution Chilo partellus (Pyralidae); Control by; Tetrastichus howardi (Hymenoptera); Hymenopteran parasites; Tetrastichus howardi; Pupal hyperparasitoid performance, effects of Bt maize, bioassay Tetrastichus howardi (Eulophidae); Control of; Chilo partellus (Lepidoptera); Food plants; Zea mays; Lepidopteran hosts; Chilo partellus; Pupal hyperparasitoid performance, effects of Bt maize, bioassay; Biological pollution; Bt maize none