Abstract High performance liquid chromatography (HPLC) using fluorescent detection following post-column alkaline degradation and a sample preparation procedure for the analysis were established to quantitatively detect tetrodotoxins (TTXs) in gastropods and puffer fishes. The analysis showed 91% recovery of TTXs using the sample preparation described in this article and a wide (10-2000 ng) linear relationship between the tetrodotoxin amount and its fluorescent response. A good correlation between the results of mouse bioassay and HPLC was also obtained in this research. Each specimen of lined-moon shell, Natica lineata Roeding, collected from Tungkang from October 1994 to September 1995 was analyzed by HPLC to detect the presence and quantity of TTXs. Overall results showed that 45% of the gastropods contained TTXs with an average content of 54.5μg in toxic specimens. The rate of toxic specimens in the ten samples collected monthly varied, but there were no seasonal trends. Random sampling of cultured tiger puffer, Takifugu rubripes rubripes Temminck & Schlegel, and field-collected balloonfish, Diodon holocanthus Linnaeus, trace amounts of TTXs were found only in ovaries of balloonfishes. Muscle tissues of balloonfishes, and the livers, ovaries, and muscles of cultured tiger puffers showed no tetrodotoxin (〈 0.2 μg/g tissue) in this screening procedure. Key words: Tetrodotoxin; Natica lineata; Takifugu rubripes rubripes; Diodon holocanthus.