Condition coefficient, hepatosomatic index, and liver histology were compared between “cobalt” and normal rainbow trouts. The results showed clearly that the “cobalt” fishes had corpulent body structure and enlarged liver. From histological examinations, it was also evidenced that the liver cells in the “cobalt” fishes of smaller size (body weight: 40-60g) contained considerable amounts of glycogen, whereas the cells in fishes of larger size (160-225g) included abundant fat droplets. Therefore, the enlargement of liver in “cobalt” rainbow trout seems to be induced by this remarkable accumulation of glycogen and fat in liver cells, probably resulting from the functional impediment of this organ.