White muscardine fungus, Beauveria bassiana, was evaluated for its insecicidal activity against grubs, pupae and adults of the red flour beetle, Tribolium castaneum. In general, prolongation in grub and pupal duration was observed. As a Subsequent effect, malformed adults were evident. The adult mortality (%) increased with the increase in exposure period and conidia concentration. This proved that the time of dipping and spore concentration greatly influenced the insecticidal effects of the fungus. The bioassay revealed that adults seem to be less susceptible to B. bassiana when compared to grubs. Malformed adults were observed in pupae treated with B. bassiana.