Total dissolved organic carbon (TOC), number of colony forming units (CFU), and total direct count (TDC) were compared to the biodegradation of aniline and the number of potential degraders in water samples from head waters to down stream of the Ina River and several other sites of rivers traversing Osaka city. The results indicate that aniline degrading populations of these various microbial communities exhibit different activities probably depending on the extent of adaptation to pollutants to which the microbes are exposed. The number of aniline degraders found in river water samples was in agreement with other parameters which were used to demonstrate the degree of pollution in river water even though higher biodegradability was evident in waters which show comparatively low TOC and CFU. These results suggest that biodegradation of aniline and enumeration of its potential degraders may serve as valuable indicators for the assessment of pollution in river waters.