Studies on the germicidal activity of chlorine, bromine, and iodine were made by use of the Association of Official Agricultural Chemists official first action method for determining effectiveness of swimming pool water disinfectants. In this procedure, 0.3 ppm of available chlorine as chlorine gas has activity equivalent to 0.6 ppm of available chlorine in the buffered sodium hypochlorite control when Escherichia coli is used as the test organism. With Streptococcus faecalis as the test organism, 0.45 ppm of available chlorine as gaseous chlorine gives activity equivalent to the control. Liquid bromine at 1.0 ppm is as effective as the 0.6 ppm of available chlorine hypochlorite control with E. coli as the test organism, but 2.0 ppm of liquid bromine is necessary to provide activity equivalent to the 0.6 ppm of available chlorine control when S. faecalis is employed. With iodine as metallic iodine, 2.0 ppm is necessary to provide a result equivalent to the 0.6 ppm of available chlorine control with both E. coli and S. faecalis. In the various systems tested, gaseous chlorine was the most active form of available chlorine; liquid bromine provided the most active form of bromine, and metallic iodine provided the most active form of iodine.