Interaction between an entomopoxvirus (PsEPV) and a gregarious braconid endoparasitoid, Cotesia kariyai, in Pseudaletia separata larvae showed that infection of larvae with PsEPV was deleterious to the development and survival of C. kariyai. The survival and development of C. kariyai in PsEPV-infected P. separata larvae depended on the length of time between parasitization and viral infection. No parasitoid larvae emerged from PsEPV-infected hosts when host larvae were exposed simultaneously to parasitization and PsEPV inoculation whereas more than 80% of the hosts produced parasitoids when PsEPV was administered 5 days postparasitization. C. kariyai larvae in PsEPV-infected hosts showed a retarded development, shrank, and died about 8 days after viral exposure. Virion-free plasma from PsEPV-infected P. separata larvae was toxic to the parasitoid larvae even up to a dilution level of 32 when it was injected intrahemocoelically into the host larvae. Development of parasitoids in hosts that were simultaneously parasitized and injected with the virion-free plasm never progressed beyond the egg stage. The parasitized P. separata larvae injected with the virion-free plasma did not pupate and died within 30 days after injection.