In November, 1995, at least eight people in the Netherlands became ill after eating mussels (Mytilus edulis) cultivated at Killary Harbor, Ireland. Although human symptoms such as nausea, vomiting, severe diarrhea, and stomach cramps were reminiscent of diarrhetic shellfish poisoning (DSP), contaminations of the major DSP toxins okadaic acid (OA) and dinophysistoxins (DTXs) were very low. These observations prompted us to explore the causative toxin in the mussels for structural studies. In this paper we report the structure of azaspiracid (1), a new marine toxin having two spiro ring assemblies, a cyclic amine and a carboxylic acid, as the major causative agent of that incident.