The disinfective power of chlorine, bromine, and mixtures of chlorine and bromine at different ratios was compared. The influence of pH was also studied. The experiments were carried out in "purified" water and in natural waters of swimming pools, river, and sea. In the presence of high amounts of nitrogenous growth-promoting material (at neutral pH), bromine was more effective than chlorine; in waters containing low amounts of nitrogenous growth-promoting material, chlorine was found superior. Mixtures of chlorine and bromine at various ratios were found to increase in effectiveness inversely to the percentage of hypobromite generated, down to 10 or 5%. Such effectiveness was found at pH levels of 5.4 to 8.6 in both purified and natural water containing high and low amounts of nitrogenous growth-promoting material. Therefore, the above mixtures seem of practical value for the disinfection of various natural waters. Escherichia coli isolated in the presence of chlorine, either from swimming pools or after deliberate exposure to the halogen, were shown to be chlorine-resistant mutants. Their resistance was maintained for at least nine passages in the absence of the disinfectant, which accounts for the number of passages tested. Chlorine-resistant mutants were not affected by bromine alone but did show a marked sensitivity to low concentrations of bromine active in the presence of chlorine. This was achieved by admixing small amounts of bromide to hypochlorite. A hypothetical model is presented to explain the synergistic sequential block by the two disinfectants. Some chlorine-resistant mutants were found to have changed into relatively slow-growing organisms with a changed phase-sensitivity pattern.