Ciguatera is a distressing form of fish poisoning, caused by the ingestion of one or more of a series of ciguatoxins. These poisons, some of the most potent mammalian neurotoxins known, are manufactured in reef- dwelling dinoflagellates and concentrated up the piscine food chain. Human victims, not uncommon in the Pacific, Atlantic and Indian Ocean tropical and subtropical littorals, become poisoned by eating risk species of fish. The acute intoxication is clinically dramatic, resulting in paraesthesiae, dysaesthesiae, prostration, myalgia and arthralgia. In some 20 percent of cases, symptoms of fatigue, reduced exercise tolerance and non-specific aches and pains persist for months and, in a small percentage of cases, for years. Such cases would, in the absence of the prior episode of acute poisoning, satisfy the diagnostic criteria for the chronic fatigue syndrome (CFS). Occasionally, patients are encountered who have been diagnosed as having CFS because of lack of awareness of the ciguatera syndrome, but in whom in retrospect the episode of acute fish poisoning can be established. The fact that at least one potent mammalian toxin can cause a chronic syndrome indistinguishable from CFS opens the way for further research into this enigmatic condition.