Three hundred and seventy-eight passengers reported gastroenteritis during four cruises in the western Mediterranean on consecutive weeks of 1995. The rate at which cases were reported each day increased on the fourth cruise. The ship's owner commissioned an epidemiological investigation from the PHLS Communicable Disease Surveillance Centre. Cases reported explosive vomiting and diarrhoea, which lasted from 24 hours to five days, and were suggestive of viral gastroenteritis. No food handlers reported illness, but enquiries suggested that some had been ill and treated themselves. No bacterial pathogens were isolated from faecal specimens provided by cases or from water, food, and environmental samples taken from the galley. Small round structured viruses (SRSV) were identified by reverse transcriptase polymerase chain reaction in two faecal specimens and one specimen of vomit from people who became ill during the fourth cruise. SRSV was also identified in one faecal specimen by electron microscopy. Environmental inspection revealed inappropriate food handling, hygiene, and storage. During one 24 hour period no chlorine was detectable in the water. A case control study conducted on the fourth cruise sought details of exposure to various foodstuffs, unbottled water, and various parts of the ship. No significant associations were found between illness and any exposures. The evidence strongly suggested a continuing outbreak of SRSV infection transmitted from person to person. Some passengers remained on board for a second week and could have transmitted their infection to new arrivals. The ship was cleared and disinfected at the end of the fourth cruise in order to interrupt transmission. Fewer than 10 cases presented in each of the fifth and sixth cruises.