[Extract] Permethrin (3,phenoxyphenylmethyl (±)-cis, trans-3-(2,2-dichrofethenyl) 2 ,2-dimethylcyclopropanecarboxylate; C21H20Cl203); molecular weight, 391.3) is a synthetic pyrethroid which is a mixture of cis and trans isomers (Taplin and Meihking, 1996). Synthetic pyrethroids are structural derivatives of the naturally occurring pyrethrins, extracted from the flowers of Chrysanthemum cinerarifolium. In some countries, including the USA, only the 25:75 cis-trans ratio is approved for human use because the cis isomer has higher mammalian toxicity than the trans isomer (Taplin and Meinking, 1996). The dstrans composition of permethrin products not approved for human use may differ. Permethrin was formulated by Michael Elliott and colleagues at Rothampstead Experimental Station (UK) in 1973, and, although it was not the first active synthetic pyrethroid produced, its high insecticidal activity plus photostability and thermostability made it suitable for commercial development (Elliott et aI., 1973). The chemical structure of permethrin is shown in Figure 204.1.