Mortality ofHeliothis zea(Boddie) andHeliothis virescens(Fabricius) larvae after treatment withBacillus thuringiensis(Berliner) was determined in bioassays with treated cotton leaves. Larval mortality depended upon dosage, species, and size of larvae tested.Bacillus thuringiensiswas also found to reduce the weight of larvae surviving treatment (7 days) when compared with the untreated check. Bioassays failed to detect differences between two commercialB. thuringiensisformulations, Dipel (Abbott Laboratories, North Chicago, III.) and Thuricide (Sandoz, Inc., San Diego, Calif.). Effectiveness ofB. thuringiensiswas usually increased when treatments were made in combination with the commercial spray adjuvants, Coax (Trader Oil Mill Co., Ft. Worth, Tex.) and Gustol (Sandoz, Inc., San Diego, Calif.). Combinations ofB. thuringiensisand nuclear polyhedrosis viruses ofH. zeaandAutographa californica(Speyer) usually resulted in mortality levels greater than that ofB. thuringiensisalone but not greater than that of either virus alone. Advantages for combiningB. thuringiensiswith the nuclear polyhedrosis virus ofH. zeaorA. californicawere not detected in this study.1