All the summer steelheads Salmo gairdneri from the Siletz, an Oregon coastal river in which Ceratomyxa shasta does not occur, died when they were exposed to waters containing the infectious stage of C. shasta. In contrast, three strains of summer steelhead from tributaries of the Columbia River where C. shasta is endemic were resistant to ceratomyxosis. These results, together with previous work on other species, indicate that salmonids from populations living in waters free of the infective stage of C. shasta should not be stocked in waters containing the pathogen. The resistance to this disease among strains of summer steelhead from the Columbia River is believed to be a result of natural selection.