An epizootic of a myxobacterial infection in coho salmon (Oncorhynchus kisutch) was responsible for the death of 50,000 fish, 30% of the population. Cartilage in the nose, mouth and lower jaw was eroded, and yellow sheets of bacterial growth were observed in the mouth, pharynx and pneumatic duct. The severity of the disease increased with increasing water temperature. Pathogenicity trials were inconclusive; only two of 18 experimentally infected fish succumbed to the disease. However, the lesions, and the absence of other known pathogens suggests the myxo-bacterium was responsible.