The diatom communities in the River Kinokawa, a less polluted river in Japan, were surveyed in summer 1985. In almost all stations, Homeothrix janthina was dominant in algal communities on river bed. In these co mmunities, we can find Achnanthes japonica or Nitzschia frustulum var. perpusilla with high abundancy. The former occured usualy in upper stream, the latter in downstream. From the data of diatom flora in each sampling station, water quality was estimated by using Diatom Assemblage Index to organic water pollution (DAIpo ), and water quality chart was made using the indices computed. River Pollution Index ( RPID ), of which value is able to be shown the general degree of pollution of river system, was 57. The value is comparatively higher than those of other rivers in Japan.