A total of 109 cultures of Pseudomonas putrefaciens isolated from clinical specimens were studied. The cultures were separated into two groups. The majority of the group 1 isolates, comprising 31 cultures, were characterized by (i) growth in plain nutrient broth, but no growth in broth supplemented with NaCl at concentrations of 7% and above, (ii) no growth on Salmonella-Shigella (SS) agar, and (iii) production of acid from the carbohydrates, sucrose, maltose, arabinose, and dextrin. Most group 2 isolates, comprising 78 cultures, were (i) unable to grow in plain nutrient broth, but grew well in broth supplemented with NaCl at a concentration of 7 to 10%, (ii) able to grow on SS agar, and (iii) unable to produce detectable amounts of acid from any of the carbohydrates tested except for variable results with glucose and fructose.