On the basis of the facts found in the previous report that chlorine destroyed cell membrane and stalk of Caulobacter crescentus and that the damaged cells abonrmally differentiated to elongate their stalks when the chlorinated cells were reincubated in fresh medium, it was suggested that there might be some ecological correlations between the level of water pollution and morphology and phsiology of Caulobacter cells. In order to determine such correlations in this study, 18 isolates of caulobacters were isolated from three sites of Musimcheon River which were gradually polluted by domestic sewages and 6 species were identified. Four cell types of bacteroid, vibroid, subvibroid, and fusiform is were isolated from the least polluted water at the rate of 43%, whereas only two cell types of bacteroid and vibroid were isolated from the more polluted waters at the rates of 7 and 10%, respectively. There was no significant difference in each species of Caulobacter isolateed from the gradually polluted river waters, except that abnormally long filamentous cells were occasionally observed in C. crescentus isolated from the most polluted water. Each species showed various responces to chlorine. The resistances of C. crescentus isolates to chlorine were proportionally correlated with the level of water pollution. When they were compared to C. crescentus CB 13 which had been adapted to laboratory enviroment, the C. cresucentus isolated from the most polluted water was more resistant to chlorine.