Ciguatera is the commonest form of poisoning resulting from eating fish in the tropics. It has been recognised since the 15th century. The disease is due to the formation of ciguatoxin by a dinoflagellate, Gambierdiscus toxicus, loosely attached to algae growing on coral reefs. The toxin, which is harmless to the fish, is ingested by small herbivorous fish and passes up the food chain as these are eaten by carnivores. The toxic effects include gastroenteritis, itching of the skin, peripheral neuropathy and central nervous system dysfunction. Though most cases are mild, occasionally the disease is severe and even fatal. There is no effective specific treatment. Three cases, of which one died and which exhibited both peripheral and central nervous system involvement, are described. Histological changes in sural nerve fibers are described. There was striking oedema of the adaxonal Schwann cell cytoplasm. These histological changes are very similar to those seen following the injection of scorpion and spider venoms into the peripheral nerve of experimental animals. Both these venoms and ciguatoxin increase the permeability of the membrane to sodium.