Dynamics of the bacterial flora in the estuarine region of the Ohta river and the Hiroshima Bay in the Seto Inland Sea was studied from the viewpoint of bacteriological ecology. The survey of bacterial flora of the surface water from six sampling stations was carried out between September 1981 and June 1983, with six samplings done per station. The bacterial population was investigated using the MPN method. Total viable bacterial count ranges from 10⁵-10⁷/100ml. Tidal river region had higher bactelial count than the sea region. Fecal indicator organisms decrease in number towards the sea region. The organic matter decomposing bacteria e.g. sulfate-reducing, nitrate-reducing and denitrifying bacteria increase in number towards the sea. This may be due to sedimentation of organic matter in the estuarine region of the sea from the river. In every station 40 strains were randomly isolated for classification and distribution. Vibrio and Vibrio-like and Pseudomonm were found to be majority among the strains isolated. In the estuarine river region Acinetobacter and Enterobacteriaceae were the majority but deminish towards the sea, while Vibrio and Pseudomonm were few in the river region and majority in the sea. This change in bacterial flora from river region to sea region was found to be consistent throughout he survey period. The tests on the isolates showed that nitrate-reducing strains and gelatin-liquefying strains were more in the sea region than in the river region and especially the gelatinliquefying strains. This may be due to accumulation of organic substances deposited in the estuarine region by the river. These results indicate a change in bacterial flora from the river to the sea and this is presumably the direction of pollution change.