In November 1993, 188 people were admitted to hospital after eating the meat from a single shark (Carcharhinus leucas) in Manakara, a medium-sized town on the south-east coast of Madagascar. This shark and its meat had no unusual characteristics. The attack rate was about 100%. The first clinical signs appeared within 5-10 hr after ingestion. The patients presented with neurological symptoms almost exclusively, the most prominent being a constant, severe ataxia. Gastrointestinal troubles, like diarrhoea and vomiting, were rare. The overall case mortality ratio was close to 30% among the 200 poisoned inhabitants. There were no reports of previous similar poisonings in this area, and fishermen in Manakara usually eat this kind of shark without problems. Bacteriological and chemical causes were eliminated. Two liposoluble toxins were isolated from the liver and tentatively named carchatoxin-A and -B, respectively. They were distinct from ciguatoxin in their chromatographic properties.