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Investigations on infectious diseases of Pacific salmon due to micro‐organisms other than viruses are reviewed. The etiological agents include trematodes, fungi, protozoa and bacteria. Bacteria have been found to be the most important agents of disease in the several species of Pacific salmon. Kidney disease, due to a small, unnamed Gram‐positive diplobacillus, causes serious mortalities in young salmon reared in hatcheries. The disease has also been found in wild fish. Aquatic myxobacteria are important agents of disease both in the hatchery and in the natural habitat. One of the myxobacteria, Chondrococcus columnaris, causes disease at relatively high water temperatures. The problem of the taxonomy of this organism is discussed. Another myxobacterium, Cytophaga psychrophila, has been found responsible for epizootics in coho salmon at lower water temperatures, i.e., in the range of 40° to 55° F. In outbreaks of gill disease in young salmon, myxobacteria of several kinds have been implicated. A variety of bacteria has been found responsible for outbreaks of disease in salmon in sea water. The most important of these is a species of Vibrio. Tuberculosis has been found in adult chinook salmon and the evidence indicates that the disease was contracted at sea.

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Lysed areas suggestive of phage activity were observed on a plate in the course of isolation of a strain of Chondrococcus columnaris from trout at the Fisheries Center of the University of Washington. C. columnaris is an aquatic fruiting myxobacterium responsible for epizootics in salmonid and other fishes (Ordal and Rucker, 1944). Subsequently a bacteriophage was isolated which infected and lysed this particular strain of C.