We have begun to evaluate the roles of naphthyl esterase and insensitive acetylcholinesterase enzymes in conferring resistance to insecticides in California populations of Lygus hesperus. Initial work has focused on an alfalfa population in Moreno Valley which has served as a useful source of insects for developing the assays we hope to use in more extensive studies during the Summer of 2001. Naphthyl esterases were characterized using well established methods of polyacrylamide gel electrophoresis. Our preliminary data has shown that the resolution of Lygus naphthyl esterases using a barbitone buffer system was poor; this is in contrast to the excellent resolutions obtained with the same protocol for other insects such as whiteflies and aphids. We identified one form of acetylcholinesterase (AChE) in this population and this has now been well characterized using spectrophotometric techniques. Km and Vmax for this AChE have been determined and we have preliminary data showing it to be highly sensitive to inhibition by the organophosphorus (OP) insecticide, chlorpyrifos-oxon. For toxicological assessments, a filter-paper bioassay method was developed and tested. Its use enabled direct comparisons between bioassay LD ₅₀ data and manufacturers’ recommended field application rates.