Morphological and ecological studies on two forms of Oncorhynchus rhodurus Jordan et McGregor living in Lake Biwa and adjoining inlets were conducted. The fluviatile form (the amago) and the lacustrine form (the biwamasu) showed morphological differences in number of pyloric caeca, transverse scales, ventral fin rays and red spots on lateral body. The fluviatile form lives in the upper waters of inlets to Lake Biwa, but the lacustrine form leaves the streams as fry and lives in Lake Biwa almost all its life. In addition to this, there are ecological differences in maturity age, growth rate and food habits between the two forms. It is presumed that the fluviatile form and the lacustrine form living in this lake and adjoining inlets consist of different populations.