Abstract: Economic losses caused by the storage product pests are considerable worldwide. The use of synthetic insecticides is limited due to the increased regulatory constraints and pest resistance. Biopesticides based on the δ-endotoxins (Cry proteins) from Bacillus thuringiensis are promising but some storage product pests show low sensitivity. To clarify the underlying mechanisms of such resistance, Tenebrio molitor (susceptible species) and Tribolium castaneum (resistant species) neonates were fed diet treated with Cry3Aa with subsequent analysis of mortality, fitness and in vivo Cry3Aa processing. LC50 of Cry3Aa for T. molitor was 110.2 mg/kg (after seven weeks of exposure). The sublethal Cry3Aa effects were manifested by the reduction of larval weight and increase of the developmental time. Tribolium castaneum tolerated the Cry3Aa doses as high as 2500 mg/kg. Such a difference in sensitivity between two closely related tenebrionid pests may be attributed to a difference in their life history strategies (faster development of T. castaneum). However, differences in Cry3Aa processing in the midgut lumen of both pests cannot be excluded. Analysis of Cry3Aa proteolysis in T. castaneum larval midgut revealed its efficient processing.