Peracid activated organophosphorus (OP) insecticides were evaluated against electric eel and a range of insect acetylcholinesterases (AChE). Incubation of chlorpyrifos and sulprofos with meta-chloroperoxybenzoic acid (MCPBA) produced active metabolites capable of distinguishing between AChE enzymes conferring OP susceptibility and resistance in populations of the tobacco whitefly, Bemisia tabaci Gennadius, and the beet armyworm, Spodoptera exigua Hubner. These products were also active against enzymes present in the lygus bug Lygus hesperus Knight and the greenhouse whitefly, Trialeurodes vaporariorum Westwood. Although profenofos was active against electric eel AChE, incubation of the OP with MCPBA did not enhance its anti-AChE properties toward the eel or insect AChEs.