Two forms of acetylcholinesterase (AChE) were identified in field populations of the beet armyworm, Spodoptera exigua (Hubner) collected from cotton in San Joaquin Valley, California. Strains (BESS and BKRR) homogeneous for each variant were isolated and their relative susceptibilities to methomyl, chlorpyrifos and chlorpyrifos-oxon assessed by topical application bioassay. In comparisons with a laboratory susceptible strain (DOW), BKRR and BESS expressed 68-fold and 7-fold resistance, respectively, to the carbamate methomyl. Neither strain was cross-resistant to chlorpyrifos or its oxygen analog (chlorpyrifos-oxon). In biochemical studies, the BKRR AChE enzyme was ca. 30-fold and 7-fold more insensitive to methomyl and chlorpyrifos-oxon, respectively, compared with both the DOW and BESS enzymes. The correlation between the toxicological and biochemical studies provides strong evidence that target-site insensitivity is the predominant mechanism of resistance to methomyl. The lack of significant cross-resistance to chlorpyrifos suggests also that the insensitive AChE in these field populations was selected by methomyl alone and not by the OP.