Whole body homogenates from azinphosmethyl-resistant fifth instars of the tufted apple bud moth demonstrated 11.8-fold elevated phosphoric triester hydrolase (methyl paraoxonase) activity as compared to susceptible insects of the same species. Elevated phosphoric triester hydrolase (PTEH) activity associated with resistance was also found in the Colorado potato beetle but not in the German cockroach or tobacco budworm. Phosphoric triester hydrolase activity in the tufted apple bud moth was minimal in resistant and susceptible third instars and in adult males and females and was highest in whole body homogenates and in the alimentary canal of resistant fifth instars. A microtiterplate assay was developed, which successfully diagnosed resistance in individual fifth instars based on increased phosphoric triester hydrolase (methyl paraoxonase) activity. Phosphoric triester hydrolase was purified 289-fold from fifth instars of resistant bud moths, but any additional resolution resulted in the loss of enzyme activity. Phosphoric triester hydrolase demonstrated an apparent molecular weight of 41,000 with an isoelectric point of 5.28. Methyl paraoxonase activity was increased by calcium, cobalt, manganese, and octylthio-1,1,1-trifluoro-2-propanone and decreased by mercury, phosphate ions, tin, and ethylenediaminetetraacetic acid. Iron, potassium chloride, lithium, magnesium, sodium chloride, and lead had no effect.