The ultrastructure of Prototheca salmonis is described, and compared with known isolates of Prototheca. Prototheca salmonis was found to be the causative agent of a disease, grossly affecting the visceral organs of Salmon parr in fresh water stocks of a fish farm. The organism was distributed widely throughout the visceral organs of the diseased fish. Prototheca salmonis, was also isolated from the infected tissues and grown in culture. The ultrastructure of the organism showed many similarities with the ultrastructure of the known isolates. The notable differences were the structure of the cell wall, the presence of glycogen, and the absence of plastid-like organelles, which were a prominent feature of the known isolates of Prototheca.