A dot-blot immunoassay, using antiserum raised against esterase B1 responsible for organophosphate (OP) resistance in Culex quinquefasciatus, was used to study different laboratory strains and field collections of this species, as well as of Cx. pipiens, Cx. tarsalis, Anopheles albimanus and An. stephensi. The frequency of esterase B1 positive individuals revealed by this test was in agreement with the data obtained by other methods, indicating the potential of this method for the detection and monitoring of resistance due to increased detoxification by esterases.