The activity of the nucleopolyhedrovirus of the fall armyworm, Spodoptera frugiperda (J. E. Smith) (SfMNPV), on transgenic sweet corn, Zea mays (L.), expressing a CryIA(b) toxin from Bacillus thuringiensis (Berliner), was studied in the laboratory. As the line of Bt corn that we studied has only limited efficacy against the fall armyworm, SfMNPV was evaluated as a supplemental treatment. When fall armyworm larvae reared on transgenic or nontransgenic foliage were fed equal dosages of occlusion bodies (OB), measured either as OB per larva or as OB/mg larval weight, mortality was higher among larvae feeding on transgenic corn. However, when larvae were allowed to feed ad libitum on treated foliage, mortality was higher in larvae on the nontransgenic corn. Rates of food consumption were higher on nontransgenic corn, apparently resulting in ingestion of more OBs and countering increased susceptibility of larvae on transgenic corn.