Abstract An in vitro host-range model system, designated System II, was developed for the multicapsid nuclear polyhedrosis virus Spodoptera frugiperda (SF-MNPV). This system consisted of SF-MNPV-infected S. frugiperda cells (permissive system) and SF-MNPV-infected Trichoplusia ni cells (semipermissive system). Infection kinetic studies revealed that while progeny virus was produced in the semipermissive system, the infectious virus yield was at least 700-fold lower than that in positive controls. Electron microscopy showed that the vast majority of progeny nucleocapsids in semipermissive infections were not enveloped and were found in only 5% of infected T. ni cells; the remaining 95% of the cells exhibited cytopathic effect as well as virogenic stroma. The data suggest that the infection in System II is restricted at or prior to the early stages of virion morphogenesis.