Starch-ampicillin agar was used to quantitatively isolate Aeromonas sp. from retail grocery store produce. All produce sampled, including parsley, spinach, celery, alfalfa sprouts, broccoli, and lettuce, contained Aeromonas sp. In most instances, the count of Aeromonas sp. increased 10- to 1,000-fold during 2 weeks of storage at 5 degrees C. Eleven (92%) of 12 kinds of produce yielded cytotoxic Aeromonas sp. Identification as Aeromonas hydrophila was the strongest indicator of cytotoxicity, and all 29 (100%) A. hydrophila isolates and 1 (6%) of 16 A. caviae isolates were cytotoxic. Twenty-seven (90%) of 30 cytotoxic Aeromonas sp. strains produced hemolysins. Strong correlations were also noted between ability to produce cytotoxin and positive Voges-Proskauer, lysine decarboxylase, and sorbitol fermentation reactions. It appears that grocery store produce is a potentially significant source of cytotoxic Aeromonas sp. and should be considered in the epidemiology of A. hydrophila gastroenteritis.