The toxicity and toxin component of gastropod Niotha clathrata implicated to a food paralytic poisoning incident in Kaohsiung, Taiwan in November 2006 were studied. The highest scores of average toxicity in the digestive gland and other portions from collected gastropods were 62+/-24 (mean+/-S.D.) and 32+/-16 microg/g according to tetrodotoxin (TTX) bioassay, respectively. The toxin from these gastropods was large amount and easily identified as tetrodotoxin by traditional method of HPLC-FLD. The toxin of patient's blood serum was trace amount and analyzed by a new developed method LC-MS/MS. LC-MS/MS was contracted by the LC system interfaced with the MS/MS system with a turbo ion spray interface. Positive ion detection and multiple reaction monitoring mode were used for TTX of patient serum. It was found that linearity in serum was observed within concentration ranged of 1-100 ng/ml and limit of detection was 0.1 ng/ml. The LOQ was reproducible at 1 ng/ml in serum. The blood serum showed to contain TTX of 3.30+/-0.08 ng/ml. It indicated that LC-MS/MS was more lower detectable and believable method for TTX determination than LC-MS reported previously. Furthermore, the causative agent of gastropod food poisoning was identified as TTX.