Toxicity of a Japanese marine puffer Chelonodon patoca ("okinawafugu") was examined by mouse assay from 1996 to 1999. Frequency of the toxic specimens was found to be 100% with high toxicity scores. Among the tissues tested, toxicity in the skin ranged from 60 to 6,700 MU/g, in the ovary from 25 to 670 MU/g, in the testis from 45 to 550 MU/g, in the muscle from 2 to 390 MU/g, and in the liver from 5 to 380 MU/g. The liver, which is known as one of the most toxic organs in Japanese marine puffer in general, showed lower toxicity in the present study. Thus, the anatomical distribution of toxicity was unique in C. patoca, in comparison with that of other Japanese puffers. C. patoca toxin was characterized as tetrodotoxin (TTX), 4-epiTTX and anhydroTTX by HPLC.