Abstract: Red flour beetle, Tribolium castaneum, is one the most important and worldwide pest of stored-products. To evaluate the susceptibility of different life stages of this pest we used a commercial formulation of biorational insecticide, Spinosad, Tracer® against adults, young and old larvae and pupae using topical application bioassay method. Mortality was recorded after exclusion of pupae and 24, 48 and 72 h post-treatment for the other life stages. Spinosad did not cause pupal mortality. Comparison of LD50 values of different life stages of T. castaneum at 48 h exposure-time revealed that, young larvae and adults were more susceptible to spinosad than the old larvae. Overlapping of 95% CL revealed similarity of susceptibility of young larvae and adult insects to spinosad. Due to compatibility of spinosad with environment, this compound could be considered as a useful tool in the control of the pest in question.