The report discusses denitrification of waste water. The denitrification was accomplished by continuous flow reactor, a gravity solids-liquid separator, and sludge recycle. A portion of the raw waste-water was added as an external carbon source. By operating two completely mixed biological reactors in series it was possible to accomplish both nitrification and denitrification. Aeration times of 7-9 hours were used to produce a completely nitrified effluent at temperatures as low as 10-12C. Denitrification was accomplished with a 1.3 hour mean reaction tank detention time at 20C and in 2.6 hours at 10C. Effluent ammonia nitrogen was that amount which was added to provide a carbon source. Therefore, addition of wastes with little or no ammonia nitrogen as a carbon source would produce an effluent nearly devoid of ammonia nitrogen. The BOD to nitrogen ratio was shown to be the critical parameter, inasmuch as high ratios would produce a high effluent carbon content, and low ratios resulted in complete denitrification. Keeping the ratio constant resulted in an effluent nitrogen concentration well below 1.0 mg/1. By this same analysis, the rate of nitrate reduction was shown to be a linear function of the sludge loading rate. (WRSIC abstract)