A nonoccluded virus (identified as anAscovirus), isolated from fall armyworm (FAW),Spodoptera frugiperda(J. E. Smith), was tested to determine its compatability with the parasitoidCotesia marginiventris(Cresson). AdultC. marginiventrisreadily transmitted the virus from infected to noninfected FAW larvae. However, the progeny of the parasitoids failed to complete development in host larvae inoculated with the virus before or during parasitization. Fewer parasitoids completed development in host larvae inoculated 4 days after parasitization than in larvae inoculated 5 days after parasitization and in parasitized controls. Mortality was not due to the virus killing the host before the parasitoids could complete development, since the virus-infected larvae lived longer than the parasitized controls. Fewer and smaller parasitoid larvae were found in virus-infected hosts than in parasitized controls when host larvae were dissected 6 days after parasitization.