The pathway of Autographa californica nuclear polyhedrosis virus (AcNPV) infection in cabbage looper, Trichoplusia ni, larval midgut cells was studied by ultrastructural and virus titration methods. Enveloped virions interacted with microvilli of columnar cells resulting in apparent fusion of the viral envelope and microvillus membrane. After entry into the cell cytoplasm, the intact nucleocapsids appeared to enter the nucleus through nuclear pores, and uncoating of the viral genome took place in the nucleoplasm. Viral progeny were first observed at 8 hr postinoculation (p. i.) and the developmental cycle of the virus was essentially completed by 24 hr p.i. Inoculum virus nucleocapsids also moved to the basal plasma membrane and budded into the hemocoel through the basal lamina within 0.5 hr p.i. We propose that this budded virus, possessing an envelope with a peplomer structure, is the primary inoculum for the systemic invasion of the insect host.