An invasive strain of Escherichia coli (ONT:NM) was isolated from stool specimens from 7 of 10 ill passengers who developed diarrhea during a 5-day ocean cruise. The ill passengers had shared no common exposures off the ship before or during the cruise. Three of the persons whose stools were cultured were part of a tour group of 219 persons, and a food consumption and health history questionnaire was completed by 190 members (87%) of this tour group. Forty-seven (25%) had had diarrhea during the cruise; other symptoms among those with diarrhea included nausea (72%), abdominal cramps (68%), headache (68%), chills (60%), dizziness (53%), myalgias (43%), subjective fever (36%), and vomiting (26%). The median duration of symptoms was 3 days. Eating at cold buffets on ship and eating potato salad, a buffet food item, were significantly associated with illness. No evidence of secondary spread of illness in household contacts of the ill person was found.