Mosquito survival in the aquatic environment is highly dependent upon the nature of the air-water interface. Nonionic monomolecular organic films reduce surface tension and increase the wettability of larval and pupal breathing structures, thereby causing high mortality. Enclosing adults and those alighting on a film-covered surface are readily wetted and drowned. The most effective and persistent agents are spontaneously spreading, surface-active liquids which reduce the aqueous surface tension to below 29 dynes/cm. In addition, the films displace natural scums and are self-healing after being disturbed by wind and waves. The films are effective in a natural paludal environment at a surface concentration of 0.04 milliliters per square meter (0.043 gallon/acre). Optimum properties of film-forming materials for the practical control of mosquitoes are itemized and related to chemical structural considerations.