Ceratomyxosis is caused by the myxosporean Ceratomyxa shasta and is an important pathogen of salmonids on the Pacific northwest coast of North America. It can cause serious losses in both cultured juvenile fish and wild prespawning adult fish, although different strains of the same species may vary in their susceptibility to the disease. C. shasta infects all layers of intestinal tissue while susceptible species of anadromous salmonids are in their freshwater stage. Other infected tissues can be gill capillaries, spleen, kidney, gonads and liver. Exposure to contaminated sediments or water can induce infection at temperatures higher than 10°C. Clinical signs vary depending on the species but general external symptoms include anorexia, lethargy, ascites and exopthalmia. Internally the intestinal tract can become haemorrhaged with mucoid contents. The most effective control measure is avoidance of water supplies containing the infective stage or the introduction of resistant salmonids.