Frenette,-C.; MacLean,-J.D.; Gyorkos,-T.W.J. A large common- source outbreak of ciguatera fish poisoning. -INFECT.-DIS. 1988. vol. 158, no. 5, pp. 1128-1131. Ciguatera fish poisoning is being reported with increasing frequency in Canadian vacationers returning from the Caribbean. It is probably the most common form of poisoning resulting from eating fish in the tropics. The disease is due to the production of a toxin by a dinoflagellate, Gambierdiscus toxicus , that is found loosely attached to algae growing on coral reefs. The toxin, which is harmless to the fish, is ingested by small herbivorous fish and passed up the food chain by larger carnivorous fish. The toxin is heat stable and is found in the flesh of > 200 species of large fish, including grouper, barracuda, red snapper, amberjack, and kingfish. Several large case series have been reported in the literature, but most of these have been retrospective reviews of small outbreaks occurring over a period of several years. In the present study, the authors report a large outbreak of ciguatera fish poisoning among Canadian tourists in Cuba; the outbreak resulted from a single common fish meal eaten on 27 March 1987. The goals of their study were to determine the clinical spectrum and duration of disease and to estimate the morbidity associated with this outbreak.