We investigated the effects of interaction between Cotesia plutellae (Kurdjumov) and Bacillus thuringiensis Berliner on parasitism and an immune effector (phenoloxidase activity) of a Bt-susceptible and a Bt-resistant population of Plutella xylostella (Linnaeus) in laboratory. Parasitism success of C. plutellae varied depending on the use of B. thuringiensis or its toxin, and the timing of application. Percentage parasitism was significantly greater on Cry1Ac-treated hosts than B. thuringiensis var. kurstaki-treated hosts (the susceptible population), and greater when hosts were treated with B. thuringiensis var. kurstaki before parasitism compared to that after parasitism (the resistant population). Specific phenoloxidase activity was significantly reduced in Cry1Ac-treated or parasitized hosts, but combined effect of the toxin and the parasitoid caused a greater reduction in phenoloxidase activity. The lower phenoloxidase activity in unparasitized resistant population of P. xylostella compared with the susceptible one is likely to be due to fitness costs, manifesting a possible trade-off between pathogen resistance and parasitoid resistance. However, C. plutellae overwhelmingly suppressed phenoloxidase activity of both the susceptible and resistant populations of P. xylostella. We found that the interaction between B. thuringiensis and C. plutellae was synergistic, which is promising for integration of the pathogen and the parasitoid in management of P. xylostella populations.