To determine the role of Vibrio cholerae as a cause of diarrheal illness in Cancun, Mexico, an investigation was conducted in July and August 1983. Although toxigenic V. cholerae 01 were not found, non-01 V. cholerae were isolated from 22 (16%) of 134 stools from persons with diarrheal illness and none of 22 stools from well persons; 58 (92%) of 63 sewage samples; 12 (86%) of 14 untreated well water samples; a home storage tank for treated water; and 5 (21%) of 24 samples of raw seafood. None of the V. cholerae isolates from patients were toxigenic. The illnesses occurred mainly in small children, and were characterized principally by diarrhea and abdominal pain. No patient was seriously ill, and all recovered without sequelae. Seven different serotypes of non-01 V. cholerae were isolated from the stool specimens, and Smith serotype 12 accounted for 10 (46%) of the 22 isolates. A matched-pair case-control study found that cases were more likely than controls to have eaten home prepared gelatin (P = 0.03, OR = 5/0) and seafood (P = 0.06, OR = 4/0).