Surf clam Pseudocardium sachalinensis was collected from coastal waters of Ibaraki Prefecture and dissected into various parts, which were analyzed for paralytic shellfish poisons (PSPs) by HPLC. In all the parts, PSP was composed almost exclusively of protogonyautoxin (PX) and gonyautoxins (GTXs) 1-4. The relative amounts of those components, however, differed among the parts. Overall, the ratio of GTX (1, 4) (N1-OH carbamate toxins) to GTX (2, 3) (N1-H carbamate toxins) clearly differed between the visceral parts and the muscle parts, suggesting the involvement of enzymatic reaction. When added to the visceral extract, GTX (1, 4) (N1-OH toxins) was for the most part converted into GTX (2, 3) (N1-H toxins). This, along with some other data, suggested that biotransformation of GTX (1, 4) to GTX (2, 3) takes place in surf clam viscera.