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We investigated the possibility of microorganisms contributing to reduced toxicity of ovary from puffer fish. Puffer fish ovary is fermented in salted rice-bran paste in Ishikawa Prefecture, Japan, and is one of the special marine products of this district, though fresh ovary seems to be tainted by puffer fish toxin, tetrodotoxin. A portion of liquid was taken from a vessel of rice-bran paste during the manufacturing process and known amounts of tetrodotoxin were added and their toxicities were measured. Also, 185 isolates from the products during the process of manufacturing were incubated in broth media containing tetrodotoxin and toxicity was measured after two-weeks incubation. The products were directly added to a medium containing tetrodotoxin, and toxicity and microorganisms were also investigated. A distinct toxicity decrease originating from the rice-bran-paste microorganism was not confirmed. On the other hand, the mouse bioassay method of evaluating the toxicity of known amounts of tetrodotoxin added in the fermented puffer fish ovary showed low recoveries (50-75%) of added amounts.