A new type of seafood toxicity, called amnesic shellfish poisoning, was described from 107 human cases after individuals consumed mussels containing domoic acid harvested from Prince Edward Island, Canada, in 1987. Most of these cases experienced gastroenteritis, and many older persons or others with underlying chronic illnesses developed neurologic symptoms including memory loss. Standard treatment procedures for the neurologic condition were not effective and three patients died. Domoic acid is a known neurototoxin, and it is believed that in these cases enough toxin was absorbed through the gastrointestinal system to cause lesions in the central nervous system. The most severely affected cases still have significant memory loss 5 years after the incident. The source of the domoic acid was identified as the pennate diatom, Nitzschia pungens f. multiseries , which was ingested by the mussels during normal filter feeding. A possible biosynthetic pathway for the toxin has recently been determined. Certain marine macroalgae also contain this toxin but have no association with human illness. Domoic acid, produced by N. pseudodelicatissima , has been found in shellfish in other eastern Canadian locations. In addition, domoic acid was identified in anchovies and pelicans in Monterey Bay, California, the source of which was Pseudonitzschia australis . In November, 1991, domoic acid was found in razor clams and crabs harvested in Washington and Oregon States and may have caused human illness from ingestion of the clams. Control mechanisms have been put in place in Canada to prevent harvesting of the shellfish at ≥20 μg/g, and no further human illness has been reported since the 1987 episode.