Tobacco budworm,Heliothis virescens(F.), and corn earworm,H. zea(Boddie), were reared on diets containing various concentrations of the multiple-embedded, nuclear polyhedrosis virus (NPV) fromH. armigera(Hübner) andBacillus thuringiensis(Berliner) (B.t.) (HD-1 isolate), both individually and as mixtures. Addition of NPV did not significantly affect the 7-day mortality due to normally lethal concentrations ofB.t., but the addition ofB.t.to normally lethal concentrations of NPV reduced mortality. An apparent synergistic relationship occurred between the pathogens at a normally sublethal concentration.H. virescenslarvae fed on NPV were smaller at all concentrations tested, but their weight was not related to concentration of the virus. Weight of survivingH. zealarvae was affected only by high concentrations of NPV. Normally sublethal concentrations ofB.t.affected larval weight of both species, and the weights were inversely proportional to the concentration ofB.t.