Mycobacteriosis was found in mountain whitefish (Prosopium williamsoni) taken from the Yakima River near Richland, Washington in 1975 and 1976. The disease appeared to affect about 8% of the population sampled. Gross lesions were present in most visceral organs, but were most common in the kidney, liver and pyloric caeca. Microscopically, the lesions consisted of large numbers of macrophages containing numerous intracellular bacilli. An organism was isolated and has been tentatively classified as Mycobacterium sp., Runyon group III.