Between June 23 and June 30, 1973, 90% of 650 passengers and at least 35% of 299 crew members experienced a diarrheal illness during a 7-day Caribbean cruise aboard a passenger cruise liner. Symptoms were consistent with shigellosis, and Shigella flexneri 6, Boyd 88 biotype, was isolated from rectal swabs taken from 8 to 35 ill passengers and 33 of 294 crew members. Epidemiologic evidence incriminated the ship's water, including ice, as the probable vehicle of transmission, and elevated coliform counts were found in potable water samples obtained aboard the vessel at the peak of the outbreak. Potential sources of contamination of the vessel's potable water supply were investigated, and improvements in the loading and chlorination of potable water were recommended.