The genera Aeromonas and Plesiomonas are gram-negative, cytochrome oxidase-positive, asporogenous, facultatively anaerobic rods that ferment carbohydrates with the production of acid or acid and gas. They are members of the family Vibrionaceae. Members of these genera are motile by means of polar flagella but in many other ways, resemble organisms belonging to the Enterobacteriaceae. Three species of Aeromonas are currently recognized in the eighth edition of Bergey's Manual, based on the studies of Schubert. A. salmonicida, a pathogen in fish which is non-motile and incapable of growing at 37°C, A. punctata, an aquatic environmental isolate possessing two subspecies and A. hydrophila, an organism associated with a wide range of clinical manifestations including wound cellulitis, septicemia, acute diarrheal disease, urinary tract infections and rarely, endocarditis, neonatal meningitis, otitis, peritonitis and osteomyelitis. In 1976, Popoff and Vern described a new species of Aeromonas A. sobria, which while motile, differed from A. hydrophila by a lack or significant decrease in the presence of several enzymes such as elastase, lipase and B-glucosidase. This organism represented a minority of environmental strains in a survey conducted by Seidler but seemed to predominate in a series of clinical isolates reviewed by Daily and colleagues in which the majority of Aeromonas isolates studied were A. sobria, characteristically cytotoxic, enterotoxigenic and lethal for mice. Unfortunately, since many clinical laboratories have not differentiated A. sobria from A. hydrophila in the past, the true incidence of this organism in human infections is largely unknown. Taxonomic studies performed by Maclnnes and Trust suggested that A. sobria should be included as a biotype of A. hydrophila.