During a 20-month period 55 strains of Aeromonas species were isolated from 53 children with diarrhea. The isolation rate of 2.5% for Aeromonas compared with the rates of 4.5% for Shigella, 3.3% for Salmonella, 2.7% for Campylobacter and 0.05% for Yersinia. In 45 children Aeromonas was the sole bacterial enteropathogen identified. Aeromonas was also isolated from 2 (0.5%) of 380 asymptomatic children. Despite its known lack of identifiable virulence properties, Aeromonas caviae was the most prevalent species, accounting for 69% of the isolates. None of the A. caviae strains produced cytotoxin by the 51Cr release assay and 12.5% were weakly enterotoxigenic by the infant mouse assay. All of the Aeromonas sobria and 71% of Aeromonas hydrophila were positive for both toxins. Ninety-two percent of the children with Aeromonas-associated diarrhea were younger than 3 years; 84% of the cases were seen between May and October. The majority of the children had an acute onset of watery diarrhea. Fever and vomiting were most commonly associated with the isolation of A. sobria. Eight children had chronic or intermittent diarrhea lasting for weeks to months before consultation; A. caviae was the isolate in all these cases. Several complications possibly related to Aeromonas intestinal infection were observed. These included Gram-negative bacteremia, intussusception, internal hernia strangulation, hemolytic uremic syndrome and failure to thrive in patients with chronic diarrhea.