The two 125-mgd oxygen activated sludge wastewater treatment plants for the City of Indianapolis, Indiana use ozone disinfection. The process was constructed in 1983, and has maintained continuous, reliable operation since 1985. Disinfection is required at Indianapolis from April 1 through October 31. Equipment performance characteristics were evaluated during the 1985 disinfection season, and disinfection performance characteristics were optimized during the 1986 season. The evaluation documents the effect of cooling water temperatures on equipment performance, and the utilization of ozone contactor off-gas control of disinfection performance. The capital cost of both ozone systems represented about eight percent of the plants' total construction cost. The ozone system O & M cost represents about 1.9 percent and 3.7 percent of the total plant O & M cost at the Belmont and Southport plants, respectively.