Abstract We used four organophosphorus compounds (paraoxon, dichlorvos, naled, and oxydemeton-methyl) to study the inhibition of acetylcholinesterase purified from five field populations (Caldwell, Kuna, Roswell, and Star in Idaho and Logan in Utah) of Lygus hesperus Knight. The sensitivity of acetylcholinesterase to the four organophosphorus compounds in different insect locations was Logan > Roswell > Caldwell > Kuna > Star, and the insensitivity spectrum of acetylcholinesterase to different organophosphorus compounds was rather broad. A good correlation between the specific activity of carboxylesterases and trichlorfon LC 50 among the five populations of L. hesperus suggested that trichlorfon resistance was mainly established by carboxylesterase activity. Preincubation of paraoxon, dichlorvos, or oxydemeton-methyl with carboxylesterase (197.5 units/mg protein) from the Roswell insects reduced the inhibitory potency of these compounds by 81, 51, and 22%, respectively, while preincubation with carboxylesterases (44.7 units/mg protein) from the Logan insects reduced the inhibitory potency of these compounds by 33, 39, and 4%, respectively. Carboxylesterases in L. hesperus were fairly specific to certain organophosphorus compounds. It could reduce paraoxon more efficiently than oxydemeton-methyl.