Eighty-seven populations of California red scale, Aonidiella aurantii (Maskell), from the San Joaquin Valley of California were tested for insecticide resistance by using chlorpyrifos, methidathion, and/or carbaryl in a standard fruit-dip bioassay as well as for general esterase activity by using alpha-naphthyl acetate as a substrate in a colorimetric test. The percentage of individuals that survived a discriminating concentration of methidathion, chlorpyrifos, or carbaryl was significantly correlated with the percentage of individuals showing > 0.4 nmol of esterase activity per minute per microgram of protein in the colorimetric test. Scale survival of the organophosphates showed a higher correlation with esterase activity than survival of carbaryl. These results suggest that the colorimetric test of esterase activity is useful as an indicator of the frequency of organophosphate-resistant and, to a lesser extent, carbamate-resistant individuals in California red scale populations. The results of tests for activity and inhibition of acetylcholinesterase activity suggest that California red scale is using increased amounts of esterase enzymes, including acetylcholinesterase, to sequester organophosphate and carbamate insecticides, rather than modified acetylcholinesterase. Third instars collected from twigs, leaves, and fruit showed similar levels of esterase activity. The colorimetric test of esterase activity is a useful tool to detect organophosphate and carbamate resistance in San Joaquin Valley California red scale because of its speed of testing over a wide range of months, allowing for within-season decision making by citrus growers.