Viable counts were determined seasonally on the intestinal contents of 55 masusalmon (Oncorhynchus masou), 6 white-spotted char (Salvelinus leucomaenis), and 3 kokanee salmon (Oncorhynchus nerka) collected at two rivers and two lakes in the south of Hokkaido. The microflora of their ambient waters was also determined at the same time. About 1400 strains were isolated from the above samples. The composition of the microflora was determined according to the scheme of SHEWAN et al. (1960). The results are summarized as follows: 1) Viable microbial counts of the ambient waters were less than 1 to 103 per milliliter with observed seasonal variations, i.e. higher in summer, lower in winter. The microflora included the genera Flavobacterium/Cytophaga, Pseudomonas, Aeromonas, Achromobacter, and Enterobacteriaceae. 2) Viable microbial counts in the intestinal contents of the salmon ranged from 102 to 108 per gram, with observed seasonal variations, i.e. higher from spring to summer and lower in winter. 3) The intestinal microflora was comparatively simpler than of the ambient waters. The principal genera in the intestinal contents of the salmon were the Aeromonas and the Enterobacteriaceae.