Fenvalerate is a potent insecticide that has been in use since 1976. It is an ester of 2-(4-chlorophenyl)-3-methylbutyric acid and alpha-cyano-3-phenoxybenzyl alcohol, but lacks a cyclopropane ring. However, in terms of its insecticidal behaviour, it belongs to the pyrethroid insecticides. It is a racemic mixture of four optical isomers with the configurations [2S, alphaS], [2S, alphaR], [2R, alphaS], and [2R, alphaR]. The [2S, alphaS] isomer is the most biologically active, followed by the [2S, alphaR] isomer.