In a series of laboratory experiments, larvae ofSpodoptera frugiperda(J. E. Smith) were exposed to median lethal concentrations ofS. frugiperdanuclear polyhedrosis virus (NPV), and the survivors were reared to the adult stage and mated. Larvae, pupae, and, in later experiments, adults of the second-generation (F1) insects were then examined for signs of nuclear polyhedrosis. When natural isolates of NPV were fed to third instars in the first generation, total infection rates in F1S. frugiperdalarvae and pupae ranged from o to 0.89%. When fifth instars were fed virus in the first generation, infection rates in F1S. frugiperdaranged from 3.35 to 4.32% in larvae, from 0.75 to 2.08% in pupae, and from 8.05 to 9.92% in adults. NPV was selected for an increased rate of vertical transmission by isolation of virus from infected F1pupae. The selected NPV exhibited significantly (P<0.05) better vertical transmission than did the wild isolate. When fifth instars were fed selected virus in the first generation, infection rates in F1S. frugiperdawere 7.85–12.73% in larvae, 0.73–5.93% in pupae, and 9.72–10.45% in adults. Vertically transmitted NPV caused F1mortality mainly in early-instar larvae and in prepupae and pupae; there was little F1mortality in late-instar larvae or adults.