Pastures in southeastern Louisiana were sampled to determine percent mortality caused by a nuclear polyhedrosis virus (NPV) inSpodoptera frugiperdapopulations. Data were also collected on the host populations and environmental variables. Stepwise multiple-regression analysis indicated that overwinter NPV in the soil, but notS. frugiperdapopulation density, was an important variable relative to prevalence of the disease. Time of year was another consistently important independent variable. When time was dropped from the model, a temperature variable partially compensated for its absence. Other variables that entered the models included precipitation, presence of cattle, percent ground cover, plowing of the pasture, grass height, percent silt in the soil, and host plant species in the pasture.