ABSTRACT: A food poisoning incident due to ingesting some unknown fish occurred in Hualien County, Taiwan, in December, 1992. Three persons were poisoned having symptoms which were marked by severe numbness of the lips, vomiting and difficult breathing. One of the patients died the next day. The uneaten fish meat was analyzed for toxicity. Later the fish meat showed a high toxicity (150 mouse units per gram). The fish meat toxin was purified by Diaflo YM-2 membrane ultrafiltration and Bio-Gel P-2 chromatography. The purified toxin was shown to contain tetrodotoxin and anhydrotetrodotoxin by analyses of thin layer chromatography, electrophoresis and high performance liquid chromatography. The research, by analysis, disclosed the sole causative toxic agent to be tetrodotoxin.