Between June 1982 and May 1987 Aeromonas species were isolated from 208 of 34,311 (0.61%) fecal samples submitted to a Regional Public Health Laboratory in The Netherlands. Aeromonas isolates were found most frequently in summer and rarely in winter. Of 169 Aermonas isolates that were available for further study, 19% were isolated from patients with a mixed infection, 5% from patients with underlying diseases, and 15% from patients who used medication that could predispose the intestinal tract to colonization with Aeromonas species. Aeromonas species that produced cytotoxins to Vero cells (cytotoxigenic) were found in hybridization groups 1 (11% of all isolates), 2 (1%), 3 (2%), and 8 (25%) and were identified phenotypically as A. hydrophila or A. sobria. Aeromonas species that did not produce cytotoxins to Vero cells (noncytotoxigenic) were found in hybridization groups 4 (57%) and 5A (4%) and were identified phenotypically as A. caviae. Distribution of Aeromonas species by age showed a predominance of noncytotoxigenic strains in children under the age of 5 years (46% of all noncytotoxigenic strains), while cytotoxigenic strains were mainly cultured from patients aged 50 years or older (54% of all cytotoxigenic strains). Significant correlations were found between cytotoxigenic strains and hospitalization, foreign travel, and contact with surface water. Cytotoxigenic strains were isolated significantly more often than noncytotoxigenic strains from patients with diarrhea, but in a multivariate analysis including age, previous medication, underlying disease, and foreign travel, this association was not significant.