Transmission of Microplitis croceipes (Cresson) nonoccluded baculovirus (Mc-NOBV) within and among individual Microplitis croceipes wasps was studied by light microscopy and transmission electron microscopy. Systemic transmission by way of hemolymph may represent the major route of transmission of this virus in a host. Rupture of Mc-NOBV-infected alimentary canal cells into the gut lumen suggested transmission to permissive cells and into excreta. Both sexes contributed to vertical transmission, and the percentages of virus-infected F 1 progeny from 4 combinations of healthy and infected parents. Virions were detected in the thin sections of 4 male and 7 female reproductive systems out of 10 that were examined. The percentage of horizontal transmission by female parasitization on virus injected tobacco budworms, Heliothis virescens (F.), 4 d after parasitization was 88.0%, whereas the percentage of viral infected males by feeding on 50% honey water contaminated with 1 μg/ml of virus 4 d after exposure was 92.0%.