Susceptibility of Pseudaletia separata larvae parasitized by Cotesia kariyai to an entomopoxvirus of P. separata (PsEPV) was compared to that of nonparasitized larvae under laboratory condition. Parasitized larvae were more susceptible than nonparasitized larvae when fourth-instar larvae were perorally inoculated with PsEPV and then parasitized at 3 days later. The LD-50 values for parasitized and nonparasitized larvae were 6.9 times 10-3 and 2.4 times 10-4 spheroids/insect respectively and were significantly different as indicated by lack of overlap of their 95% fiducial limits. The presence of the wasp greatly enhanced the susceptibility of Pseudaletia separata larvae to the virus infection.