A rickettsiales-like organism was isolated in cell culture from coho salmon (Oncorhynchus kisutch)reared in seawater net pens in the vicinity of Puerto Montt, Chile S.A. and experiencing an epizootic with accompanying mortality. The organism, observed in fish and isolated in cell culture, was Gram-negative, pleomorphic, predominantly coccoid in shape, and approximately 1 mu m in diameter. It replicated, producing cytopathic effect (CPE) in cell lines from four species of salmonids, but failed to grow in any of eleven standard bacteriological media. The optimal temperature for replication was 15-18 degree C based on the production of CPE. Streptomycin, gentamicin, and tetracycline was inhibitory for the organism, but it was not sensitive to penicillin.