Parasitism of larvae of the diamondback moth, Plutella xylostella (L.), by the adults of the parasitoid, Cotesia plutellae Kurdj., fed on Bacillus thuringiensis Berliner (Bt), was studied in laboratory experiments. Also, the effect of Bt feeding was studied on various biological parameters of the resultant progeny of the parasitoid. The results revealed that parasitization was reduced with Bt treatment and the effect was dose-dependent, i.e. increased with increase in Bt concentration. The impact on biological parameters of the parasitoid included reduction in pupation, adult emergence and longevity, reversal of sex ratio, and increase in pupal period. However, attributes like adult emergence and longevity were not reduced significantly at concentrations ranging from 150 to 450ppm. Similarly, when Bt-fed larvae of P. xylostella were confined with parasitoid females for oviposition, besides reduction in parasitism, cocoon formation and adult emergence were also reduced while development period was significantly prolonged. In another experiment, Bt formulations like Delfin, Dipel. Centari and Spicturin were found to be relatively safer to parasitoid adults than synthetic insecticides like quinalphos. endosulfan and fenvalerate and a neem formulation, Nimbecidine. none