We have identified 38 instances of intestinal infection by Aeromonas and 4 by Plesiomonas in 16,184 coprocultures carried out during a 4 year period in the HCU of Zaragoza. The low prevalence can be related with the fact that specific media for the recovery of these organisms were not used. The species Aeromonas sobria (47.36%) and A. caviae (39.47%) were more common than A. hydrophila (13.15%). Aeromonas were isolated from patients with and without intestinal disease. The occurrence of gastrointestinal disease was independent from the type of Aeromonas species isolated (p greater than 0.1). We did not find significant differences when we compared patients with and without diarrhea, the rates of the strains that produced gas from glucose, and those of the strains that did not produce arabinose acid. The rates of positivity for lysine decarboxylase and Voges-Poskauer were significantly more common in the strains isolated from symptomatic patients. The sensitivity to cephalothin (MIC less than or equal to 8 micrograms/ml) can be an adequate marker to differentiate A. sobria from the other two species (p less than 0.01).