Samples of fall armyworm,Spodoptera frugiperda(J. E. Smith), were collected from cornfields near St. Gabriel, La., in 1981 and 1982, and near Hammond, La., in 1982 and 1983. Prevalence of a nuclear polyhedrosis virus (NPV) was<15% at the St. Gabriel site, but exceeded 40% at Hammond. Multiple regression analysis indicated that host population factors affected prevalence of NPV at St. Gabriel, and that abiotic factors were more important at Hammond. Medium-sized larvae (third and fourth instars) were more commonly infected than small (first and second instars) or large (fifth and sixth instars) larvae. Disease did not appear sustainable in a population until a threshold density of approximately one larva per corn plant was attained.