Larvae of the common green lacewing,Chrysopa carneaStephens1, have a remarkable natural tolerance to pyrethroids, providing important selectivity in integrated control programs. A portion of this tolerance is attributable to detoxification by pyrethroid esterase(s). This larval enzyme(s) has unusually high activity and a unique specificity for hydrolyzingcis-permethrin and -cypermethrin two- to three-fold faster than the correspondingtrans-isomers. Deltamethrin is also hydrolyzed rapidly. Certain properties of the larval pyrethroid esterase(s) parallel the sensitivity of the larvae to pyrethroid poisoning. Larval pyrethroid esterase(s) increases in activity for hydrolyzingtrans-permethrin on larval growth in agreement with the increased tolerance totrans-permethrin poisoning. The relative rates of hydrolysis of deltamethrin and thecis- andtrans-isomers of permethrin and cypermethrin by the larval pyrethroid esterase(s) generally coincide with the tolerance of the larvae to these pyrethroids. Phenyl saligenin cyclic phosphonate, a potent inhibitor for larval pyrethroid esterase, synergizestrans-permethrin toxicity by 68-fold from an LD50of 17,000 μg/g to one of 250 μg/g. Although the involvement of penetration rates, nerve sensitivity, and oxidative detoxification has not been evaluated, it is clear that pyrethroid esterase(s) is a major factor contributing to the natural pyrethroid tolerance of lacewing larvae.