Deltamethrin (Deltamethrin, Decamethrin, [(S)-cyano-(3-phenoxyphenyl)-methyl] (1R,3R)-3-(2,2-dibromoethenyl)-2,2-dimethyl-cyclopropane-1-carboxylate, CAS#52918-63-5) is a pyrethroid ester insecticide. Deltamethrin is a pyrethroid composed of a single isomer of 8 stereoisomers selectively prepared by the esterification of [1R, 3R or cis]-2,2-dimethyl-3-(2,2- dibromovinyl) cyclopropanecarboxylic acid with (alpha S)- or (+)-alpha-cyano-3- phenoxybenzyl alcohol or by selective recrystallization of the racemic esters obtained by esterification of the (1R, 3R or cis)-acid with the racemic or [alpha R, alpha S, or alpha RS or + or -]-alcohol.

Deltamethrin products are among the most popular and widely used insecticides in the world^{[citation needed]} and have become very popular with pest control operators and individuals in the United States. This material is a member of one of the safest classes of pesticides: synthetic pyrethroids. This pesticide is highly toxic to aquatic life, particularly fish, and therefore must be used with extreme caution around water. Although generally considered safe to use around humans, it is still neurotoxic to humans. Deltamethrin is able to pass from a woman's skin through her blood and into her breast milk. There are many uses for deltamethrin, ranging from agricultural uses to home pest control. Deltamethrin has been instrumental in preventing the spread of diseases carried by tick-infested prairie dogs, rodents and other burrowing animals^{[citation needed]}. It is helpful in eliminating and preventing a wide variety of household pests, especially spiders, fleas, ticks, carpenter ants, carpenter bees, cockroaches and bed bugs. Deltamethrin is also one of the primary ingredients in ant chalk.