In order to clarify the processes of oxygen consumption taking place in Urado Bay, the author has made a research on the sea water and suspension paying attention to reducing matter at the same time.1. Correlative analysis of the ignition loss of suspension and the consumption of dissolved oxygen has shown that there is no significant correlation between the two.2. It is inferred, therefore, that there are different constituents in the sediment and the suspension which take part in oxygen consumption.3. The consumption of dissolved oxygen increases inwards along the center line of the bay. This fact suggests that river water may be the chief cause of pollution of the inlet.