The role of plant allelochemicals in reducing the ability ofBacillus thuringiensisvar.kurstakiBerliner to kill larvae ofManduca sexta(L.) was studied by comparing the effects of the alkaloid nicotine and the flavonoid rutin on the toxicity ofB. thuringiensis.Concentrations of nicotine and rutin reflecting levels found in tobacco and tomato plants (0, 0.2, 0.4, 0.6, 0.8, 1.0% wet weight) were incorporated into synthetic diet containing 0.001%B. thuringiensisandB. thuringiensis-free diet. Survivorship ofM. sextalarvae increased at higher levels of nicotine (0.6, 0.8, 1.0% wet weight), but higher concentrations of rutin did not increase larval survivorship. Experiments on colony growth ofB. thuringiensison nutrient agar containing nicotine or rutin (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9% wet weight) demonstrated that nicotine reduced colony growth to a greater extent than rutin. Consequently, plant allelochemicals potentially are an important factor in reducing larval mortality caused byB. thuringiensison certain crops. Specialist herbivores that can tolerate high concentrations of allelochemicals may gain protection from pathogens by feeding on plants or plant parts with higher levels of toxins.