Nuclear and cytoplasmic ultrastructural changes were examined in Spodoptera frugiperda (SF) larval fat body cells infected with granulosis virus (GV). Soon after infection necleocapsidlike structures were observed within the nucleus associated with nuclear pores. The earliest cellular alterations were the formation of intra-nuclear membrane blebs and annulate lamellae. There was also development of nuclear envelope associated crystals, followed by coalescence of heterochromatin, nucleoplasm clearing, loss of nuclear envelope integrity, paired cisternae formation, and virus assembly. Stored lipid decreased and glycogen was depleted. Mitochondria assumed a condensed configuration. Gap junctions, desmosomes, and hemidesmosomes normally present in uninfected fat body cells were lost during infection. Infected cells appeared to detach from other cells and basal lamina; concomitantly, mitotic activity was observed more frequently in peripheral uninfected cells within proximity of the delimiting fat body basal lamina than in controls. Such changes may help to explain previous observations of a proliferative response to GV infection and consequent fat body hyperplasia.