Bacillus thuringiensisvar.kurstakiBerliner andHeliothisnuclear polyhedrosis virus (NPV) formulations Dipel and Elcar, respectively, were tested in spray mixtures with several chemical pesticides: benomyl, chlorothalonil, fentin hydroxide, and thiabendazole (fungicides); methoprene and diflubenzuron (insect growth regulators); cyhexatin (acaricide); and chlordimeform (insecticide-acaricide) on cotton foliage. Foliage from treated plants was fed to first- and third-stageHeliothis virescens(F.) larvae. First-stage larval mortality response from microbial agent-chemical pesticide mixtures varied from synergistic to antagonistic.B. thuringiensiswith either acaricide, chlordimeform or cyhexatin, andHeliothisNPV-chlordimeform produced the highest mortality above the expected levels. Mixtures of either microbial agent with chlorothalonil and diflubenzuron-B.thuringiensisresulted in significantly less mortality than the expected levels. No synergism was observed when third-stage larvae were exposed to the mixtures.B. thuringiensis-fentinhydroxide or diflubenzuron andHeliothisNPV-fentin hydroxide mixtures were antagonistic.B. thuringiensis-chlorothalonil was also antagonistic when mortality was recorded at pupation.