Homologous and heterologous nucleopolyhedroviruses (NPVs) were assayed to determine their effectiveness against beet armyworm larvae, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae). Included were three NPV isolates from S. exigua, one isolate each from S. littoralis Boisduval, S. litura (F.), and S. ornithogalli (Guenée), two isolates from other noctuids, Helicoverpa armigera (Hübner) and Rachiplusia ou (Guenée), and one isolate from Plutella xylostella (L.) (Lepidoptera: Plutellidae). Before bioassays, all of the NPVs were serially passed four times through S. exigua larvae to increase their concentrations. The narrow host-range homologous SeMNPVs from South Carolina (SeMNPV-SC), Maryland (SeMNPV-MD), and Florida (SeMNPV-FL  =  Spod-X®) were three of the most effective NPVs against the homologous host, S. exigua, based on median lethal concentration values (LC₅₀). Three heterologous isolates (HearNPV from H. armigera, SpliNPV from S. littoralis, and RoMNPV from R. ou) also had low LC₅₀ values. According to their LC₅₀ values, the least active NPVs were from P. xylostella (PlxyMNPV), S. ornithogalli (SoNPV), and S. litura (SpltNPV). Based on median time to kill (LT₅₀) at LC₉₀ concentrations, SeMNPV-SC and Spod-X® were the most effective isolates. The slowest acting NPVs were from S. litura, S. littoralis, and S. ornithogalli. The virulence of homologous and heterologous NPVs did not appear to be correlated with their genomic relatedness to SeMNPV or to the relatedness of the original NPV insect hosts to S. exigua.