The penetration of contact insecticides through the cuticle of insects and the consequent lethal effect should depend on the characteristics of the cuticle at the time of treatment. Immediately after molting or after emergence of the adult from the pupa the cuticle is softer and thinner than at other times. Therefore one might suppose that insects that have just cast their skins would be more susceptible to contact insecticides than those whose cuticle has hardened and thickened. Because the literature contains little information on this question, the writers were prompted to make a few experiments on the relative susceptibility to pyrethrins of newly emerged and older blowflies, Phormia regina Meigen.