An epidemic, resulting in the death of large numbers of white crappies (Pomoxis annularis) in McKay Reservoir near Pendleton, Oregon, was found to be due to infection with Cytophaga columnaris, a myxobacterium. Extensive lesions produced by this organism were present on the body and gills of dead and dying fish. Since other epidemics are known to have been produced by C. columnaris, it is suggested that during warm weather this organism may be a frequent cause of mortalities among fish in lakes and possibly in streams.