The phenotypic characteristics of 124 strains of Alteromonas putrefaciens, a bacterium that produces large quantities of hydrogen sulfide, were studied. The strains were isolated from man, warm and cold-blooded animals (especially fish and shellfish), marine and fresh waters. Two phenetic groups were differentiated: group A strains grew at 42 degrees C, but not at 4 degrees C, tolerated 6 p. cent NaCl and did not utilize glucose or maltose, unlike group B strains. Each group could be divided into two subgroups A1, A2 and B1, B2. Citrate utilization, hydrolysis of urea and aesculin, resistance or susceptibility to cephalothin and vibriostatic agent 0/129 were useful for subgroup determination. The taxonomic relatedness of A. putrefaciens to genus Alteromonas are discussed.