The efficacy of the monomolecular surface film isostearyl alcohol containing 2 oxyethylene groups (ISA-20E) for controlling mixed populations of Anopheles quadrimaculatus and An. Crucians was evaluated in standing paludal ponds. Results of hand spray application indicated that 90-100% mortality of larvae and pupae could be achieved in 48-72 hr post-treatment at a surface dosage of 0.3-0.4 ml/m2 (0.32-0.42 gal/acre). The effects of dosage, habitat surface characteristics, dissolved oxygen levels, meteorology, and instar sensitivity on the impact of ISA-20E on immature stages of the Anopheles spp. are discussed. Observations on immature stages of Culex erraticus indicated that larvae of this species were significantly more tolerant to the surface film than larvae of the Anopheles spp. An explanation for this effect is presented. Results of preliminary tests against Anopheles and Culex spp. with automatic ISA-20E.-dispensing systems and with slow-release solid formulations of ISA-20E are also presented.