Rising water demands ask for an increasing utilization of more or less contaminated surface water that has to be chlorinated prior to treatment. Especially during periods of lake bloom, the desired disinfecting effect is not achieved in spite of high concentrations of chlorine present. This is due to a coating of bacterial surfaces by substances such as algal products having a protective effect as demonstrated in experimental studies by the authors. Bacteria in a central position within bacterial aggregates as e. g. formed by the addition of flocculants in the process of water treatment, are also protected against the action of chlorine. In cases of inadequate filter backwashing, such aggregates may penetrate into clean water. Accumulations of bacteria in the shape of widespread growth may also form on activated carbon filters, ion exchangers, at walls of tanks under unfavourable hydraulic concitions, and in stagnant parts of supply pipes. Curves depiciting the kill of dyspectic E. coli differentiated by O-agglutination and S. anatum were exhibiting longer survival periods as compared with non-aggregate bacteria. For organisms differentiated by H-agglutination, kill was between these values. The killing rate was found to be a function of the oxidation-reduction poteential. Furthermore, survival times for E. coli and S. anatum were found to be different. It has been confirmed by these experiments that chlorine concentrations as common in the practice of water works frequently do not suffice to kill bacteria within aggregates and that a colony count does not permit conclusions as to the bacterial count if bacterial aggregates are present.