The efficacy of the organic surface film isostearyl alcohol containing two oxyethylene groups (ISA-20E) for controlling Culex quinquefasciatus, Cx. nigripalpus, Psorophora columbiae and Ps. ciliata was evaluated under laboratory and/or field conditions. Results of bioassays against Cx. quinquefasciatus indicated that ISA-20E would kill larvae and pupae, entrap and drown adults, and sink and inhibit the hatching of egg rafts. Field tests conducted under a wide range of environmental conditions at surface dosage of 0.20-0.45 ml/m2 (0.21-0.48 gal/acre) indicated that effective control of larvae and pupae (90% or greater) of the Culex and Psorophora spp. would occur within 72 hr post-treatment. Factors influencing the larvicidal and pupicidal action of ISA-20E under bioassay and field conditions are discussed.