Prevalence of the nuclear polyhedrosis virus (NPV) granulosis virus (GV) was determined in populations of fall armyworm,Spodoptera frugiperda, in southeastern Louisiana. Both diseases reached their peak mean infection rates in mid-August in fall armyworms infesting pastures, the NPV at 50.8% and the GV at 2.8%. NPV prevalence in some pastures peaked at 60 to 68%. Infection rates by NPV in fall armyworms infesting corn and sorghum were initially lower than in pastures but were similar after mid-July (corn) or late July (sorghum). Infections by a mermithid nematode, an entomogenous fungus, and a microsporidium were rare.