A new differential medium, Rimler-Shotts, was tested with 109 isolates representing 13 genera of bacteria obtained from aquatic environments and animals. This medium was effective in presumptive identification of the strains of Aeromonas hydrophila examined, with 94% accuracy. Strains of Citrobacter which were hydrogen sulfide-variable could not be separated from A. hydrophila. This medium was designed to facilitate diagnosis of A. hydrophila infections in animals and humans.