The efficacy of 2 types of monomolecular organic surface films were evaluated at varying dosages against natural populations of Culex nigripalpus Theobald and Cx. quinquefasciatus Say in settling, polishing, and evapo-percolation ponds of an industrial sewage treatment plant. Results of spray application at a dosage as low as 0.33 ml/m2 water surface (0.35 gal/acre) indicted that one of the films was significantly more effective than the other formulations in controlling larvae and pupae of the Culex spp.