The synthesis of Papthion® and papoxon, optically active at the α-carbon of the α-ethoxycarbonylbenzyl moiety, was described. The optical isomers of papthion and papoxon exhibited a marked selectivity in the toxicity and anticholinesterase activity, respectively. d-Papthion was more toxic to mosquitoes, rice stem borers, diamond back moths and mice than the l-isomer. In contrast, l-papthion was more toxic to houseflies, and less toxic to mice and other insects than the d-isomer. Selectivity in toxicity appears to be directly related to the selective inhibition in vitro of acetylcholinesterase by the optical isomers of papoxon.