The main organophosphate (OP) resistance mechanism in the Culex pipiens complex is increased activity of esterases A and B. Fourth-stage larvae from 2 field populations of C. pipiens from Gaomi and Kunming, China, were compared for tolerance to parathion, dichlorovos (OP), and bassa (carbamate) insecticides. Both populations were resistant to OPs but not to bassa. Starch gel electrophoresis indicated that elevated esterase activity was correlated with OP resistance. High frequencies of amplified esterase genes B1 and A2-B2 (0.85 and 0.50) were discovered in Gaomi and Kunming, respectively. However, only low levels of gene amplification were detected.