Tetramethrin was first synthesized in 1964 and first marketed in 1965. Chemically, it is an ester of chrysan-themic acid (2,2-imethyl-3-(2,2-dimethylvinyl)-cyclopro-panecarboxylic acid) with 3,4,5,6-tetrahydrophthalimido-methyl alcohol. It is a mixture of four stereoisomers:[1R,trans], [1R,cis], [1S,trans], and [1S,cis]. In techni-cal products, the composition ratio of the isomers is roughly 4:1:4:1. Among the isomers, the [1R,trans] isomer is the most active biologically followed by the [1R,cis] isomer. A mixture of the [1R,cis] and [1R,trans] isomers (1:4) is commercialized under the trade name of `Neo-Pynamin Forte' (designated as 1R, cis/trans-tetramethrin in this monograph).