Initial monitoring of field populations of the beet armyworm, Spodoptera exigua, indicated the presence of two forms of acetylcholinesterase (AChE), the target site of organophosphorus (OP) and carbamate insecticides, using a diagnostic concentration of the carbamate, methomyl. In toxicological bioassays, the response of populations to topical applications of technical methomyl and the OP, chlorpyrifos-oxon, were related to the relative frequencies of the AChE alleles present. All populations, including a laboratory reference strain, showed a high degree of tolerance to methomyl; however, chlorpyrifos-oxon proved to be extremely potent, even against populations in which the resistant AChE variant predominated.