Sanitation being an important component of chemical-free management of stored-products pests, the nutritional stress on insects that results from a clean environment may prove advantageous to the use of microbial controls. Dietary stress by food deprivation or suboptimal diet increased susceptibility of the red flour beetle, Tribolium castaneum (Herbst), larvae to Beauveria bassiana (Balsamo) Vuillemin. Fungus-related mortality increased linearly with the number of days of food deprivation. Rearing of larvae on a rice meal diet resulted in slower development and greater susceptibility to B. bassiana than rearing on whole wheat flour with brewer's yeast. Larvae that were fed for 24 h on flour with B. bassiana conidia consumed significantly less and weighed significantly less than those that were fed fungus-free flour. Thus, the presence of B. bassiana conidia is itself a cause of dietary stress.