To find out the pattern of toxicity dynamics of snail Nassarius spp., snail samples were collected from Hanjiang, Tong’an and Xiapu of Fujian province once a week from March to September in 2006, and the toxicity of the samples was screened by the mouse bioassay method. The snail sample with the highest toxicty was then picked out and analyzed for paralytic shellfish poisoning toxins (PSPs) and tetrodotoxins (TTXs) with high performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS), respectively. It was found that most of the snail samples collected were Nassarius semiplicatus, and toxic samples were detected in all the three sampling sites during the sampling season. The ratios of toxic samples to the whole samples collected were 14%, 43% and 20%, respectively, in Hanjiang, Tong’an and Xiapu. The highest toxicity was 1620MU/100g in a snail sample collected from Xiapu on Jul. 12th. Most of the toxic samples in Hanjiang were found in March, but in the other two sampling sites, toxic samples were found both in March and in a period from June to July. It was suggested that the dynamics of snail toxicity was affected by both seasonal factors and geographical factors. There were no PSP toxins detected in the snails sample with the highest toxicity. However, TTX and its derivative, trideoxyTTX, were detected in this sample, suggesting that TTX and its derivatives should be responsible for the toxicity in snail Nassarius.