Replication of Microplitis croceipes nonoccluded baculovirus (McNOBV) in Heliothis virescens (F.) larvae was detected by double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA), transmission electron microscopy (TEM), and bioassay. Five-day-old (early third instar) H. virescens larvae were inoculated intrahemocoelically with 0 (control), 3, or 30 ng virus/larva, subjected to ELISA, and verified by TEM, at various hours after inoculation. The ELISA results showed increases of the absorbance values and the amount of virus particles produced in some inoculated larvae. McNOBV infection was observed in 16.7-33.3 and 66.7-83.3% of larvae inoculated with 3 and 30 ng virus, respectively, 24-66 h post inoculation. Virus particles were detected in hemolymph, fat body, and midgut epithelial cells of infected larvae. In bioassay, when McNOBV-infected larvae were exposed to healthy, premated, female M. croceipes (Cresson) for oviposition, 10.6% of the M. croceipes females became infected with the virus. Examination of M. croceipes offspring indicated significant decreases (control vs. infected) in larval emergence (75.0 to 39.6%), cocoon formation (68.8 to 31.3%), and adult eclosion (68.1 to 22.2%). McNOBV was detected by ELISA in 25.9% of dead larvae, 45.2% of dead pupae, and 37.3% of the surviving adults of M. croceipes. These results demonstrate that McNOBV replication in H. virescens larvae and transmission of McNOBV from infected H. virescens larvae to ovipositing M. croceipes females and their offspring can occur. none