Heavy infestations of larvae of Chironomus yoshimatsui occurred in a sewage treatment plant in Tokyo. In this plant, the final sedimentation tanks, overflow gutters, disinfective tanks, and discharge channels, having relatively low BOD levels, are the specific sites of high densities of the midge. In field studies, the relative efficacy of a WP formulation of temephos and chlorpyrifos-methyl WP against C. yoshimatsui in overflow gutters was assessed by dripping different concentrations of the larvicides for different lengths of time. Although no remarkable difference was observed in the efficacy of the two chemicals against C. yoshimatsui at treatment rates of 0.5ppm for 120min, 1ppm for 60min, 2ppm for 30min, and 4ppm for 15min, chlorpyrifos-methyl showed slightly superior activity than temephos. Successful larval control in the plant was achieved by dripping either chemical from oxidation tanks into connecting channels at a rate of 2ppm for 30min. The two larvicides and the IGR diflubenzuron applied at rates as high as 5ppm for 60min had slight impact on the function of the activated sludge in the system.