The objective of this study was to examine the effects of theB. thuringiensisbased MVP biopesticide on the development of baculoviruses inSpodopteralarvae. The sensitivity of both the beet armyworm,Spodoptera exigua(Hübner) and the fall armyworm,S. frugiperda(J. E. Smith), to the CryIA(c) toxin present in MVP was temporary; upon reaching the third instar, both species were much less susceptible to the CryIA(c) toxin. The sublethal effects of MVP onSpodoptera, reflected by a delay in larvae development, did not alter the dose response of these insects to their respective nuclearpolyhedrosisvirus. However, feeding the virus infected Spodoptera larvae MVP amended diet (10 mg/ml) did increase the number of second instars succumbing to virus infection. Microscopic examination of the larva killed by the MVP baculovirus demonstrated normal virus inclusion formation. In snmmary, onr laboratory bioassays suggested thatB. thuringiensisdelta endotoxins, applied at rates that are sublethal, should not interfere with baculovirus development inB. thuringiensistolerant hosts such asS. exiguaandS. frugiperda.