Abstract Evidence has been obtained which contradicts the widely held view that contact insecticides reach the site of action in the insect by penetrating through the integument of the body wall, subsequently being carried by the haemolymph to the target organ, the central nervous system. The present investigation shows that, on the contrary, these insecticides do not penetrate into the haemolymph in significant amounts, and that, moreover, insecticides experimentally introduced into the body cavity are less toxic than when topically applied provided that they are introduced in the absence of an organic solvent. An alternative route of entryis indicated from autoradiographic experiments with 14C-dieldrin. The label accumulates in the integument, spreading laterally therein, and reaching the site of action via the integument of the tracheal system. Measures aimed at limiting this movement were found to cause considerable reduction in toxic action.