A systemic infection with a hexamitid flagellate resembling Hexamita salmonis caused high mortality in chinook salmon Oncorhynchus tshawytscha reared at a seawater netpen farm in British Columbia, Canada. Affected fish were anemic and had swollen abdomens containing serosanguinous ascites and large blood clots. They also had an enlarged, mottled and congested liver, and an enlarged kidney and spleen. Numerous parasites were observed in the blood. The most remarkable histological changes were found in the liver and kidney. Livers of affected fish showed edema, congestion and inflammation. The renal interstitium was moderately hyperplastic due to proliferation of hemoblasts. The systemic infection was transmitted in the laboratory to chinook by intraperitoneal injection, by gavage of infected ascites and by waterborne exposure (in both fresh and sea water) with a mixture of infected ascites and tissue. The infection was also transmitted in fresh and sea water by cohabitation with infected chinook. Atlantic salmon were refractory to the infection. Based on the ease of transmission of the parasite in both fresh and sea water, and the high mortality associated with the infection, the disease poses a potentially serious threat to aquaculture of chinook salmon. (Copyright (c) Inter-Research 1992.)