Addressing whether Cry1Ab protein produced by Bt-maize affects non-target insects, including parasitoids, is a necessary component in the risk assessment of this crop protection alternative. This study assessed host-mediated effects of Cry1Ab protein on the parasitoid Cotesia marginiventris Cresson (Hymenoptera) via two delivery methods: delivery of purified Cry1Ab protein via artificial diet, and delivery of Cry1Ab protein via Bt-maize plant tissue. In the first case, lethal and sublethal effects of purified Cry1Ab protein on the host, Spodoptera frugiperda (Lepidoptera), were evaluated prior to evaluating effects on the parasitoid. Unparasitized host larvae were exposed to one of three Cry1Ab concentrations, 0.46 (C1), 9.13 (C2), and 182.6 (C3) ìg Cry1Ab/ml diet. The C3 concentration proved highly toxic to host larvae, so only host-mediated effects of C1 and C2 concentrations on the parasitoid C. marginiventris were studied. As expected, purified Cry1Ab affected survival, developmental times, and growth rates of S. frugiperda larvae at all three Cry1Ab concentrations. In contrast, host-mediated effects of purified Cry1Ab protein on C. marginiventris were not evident at the two concentrations that were evaluated, C1 and C2. However, several host-mediated effects on C. marginiventris were detected when Cry1Ab protein was delivered via Bt-maize tissue. Exposure to Cry1Ab protein via Bt-maize tissue affected parasitoid developmental times, adult size, and fecundity. Though effects on parasitoids of direct exposure (i.e. not mediated by the host) to Cry1Ab protein were not evaluated, the results of the present study suggested a direct effect of the protein, delivered via host feeding on Bt-maize, on C. marginiventris.