Chemical, physical and biological properties of a new residual insecticide namely Bayer S-1752 (29 493) (O,O-dirnethyl O-(4-methylthio-m-tolyl)phosphorothioate) are described. Under laboratory conditions residues of S-1752 were tested on glass plates and sun-dried mud against flies (Musca domestica L.), triatomids (Rhodnius prolixus Stahl) and mosquitoes (Culex p. quinquefasciatus Say). Varying exposure and observation times according to the testing animals were used. The results showed that the duration of persistence of S-1752 depends upon the absorptivity of the treated surface. No difference in efficacy of S-1752 was found against susceptible or DDT-resistant house flies.