Frass of late-instar larvae ofTenebrio molitor L. contained 0.0889 g of butyric acid, 0.0279 g of propionic acid, and 0.0175 g of valeric acid per 100 g. Grouped larvae were strongly repelled by butyric acid at the 10(-1) M concentration. Lower concentrations of butyric acid were less repellent. Valeric acid was repellent at 10(-1) M to 10(-3) M concentrations, below which no tested concentration, including one identical to that occurring in prepared solutions of frass found to be attractive, displayed any effect. Propionic acid was repellent at the concentration in prepared solutions of frass, which were strongly attractive. Concentrations of 10(-1) M, 10(-3) M, and 10(-4) M propionic acid were also found to be repellent. The implications of the repellency of these compounds to groups of mealworm larvae are discussed, with particular reference to the interaction between these frass components and others that have already been studied.