Shewanella putrefaciens, a respiratory facultative anaerobe isolated from the Black Sea, can reduce thiosulfate, sulfite, and elemental sulfur to sulfide readily and quantitatively. This widespread and anaerobically versatile microorganism, which is incapable of reducing sulfate, uses oxidized sulfur intermediates as electron acceptors during the respiratory oxidation of organic matter. Because of its widespread distribution and abundance, it may play a significant role in sulfur and trace metal cycling in the Black Sea and in other marine and freshwater anaerobic environments.