Attempts were made to elucidate the responsible toxins in the gastropod Polinices didyma which recently caused food poisoning incidents in western Taiwan. The remained (cooked) and captured (live) specimens of both gastropods were assayed for toxicity (as tetrodotoxin = TTX). Average toxicity of cooked and live specimens was 118 ± 105 and 47 ± 28 MU/specimen, respectively. In addition, another species Natica lineata collected from the same area was also found to be toxic. The toxin of each gastropod was partially purified from the methanolic extract of the gastropod by ultrafiltration and Bio-Gel P-2 column chromatography. HPLC and GC-MS analyses demonstrated that the toxin consisted of TTX. It was concluded that the causative agent of the above food poisoning was TTX.