In order to aim the prevention of the radiation hazard to human beings through sea food, the accumulation and excretion of 65Zn by octopus Octopus vulgaris was examined by the radioisotope tracer experiment. The concentration factor of 65Zn for whole body of the octopus that take up the nuclide from sea water and food was estimated as 9, 900, by assuming that the octopus feeds on clams alone. In that case the contribution of food was about twenty times greater than that of sea water on the accumulation of the nuclide. The biological half-life of 65Zn accumulated through sea water was 74 days. High accumulation of 65Zn in the branchial heart of the octopus, as in the case of Co, was not observed. In the liver, 65Zn combined with three constituents which have a molecular weight of more than 80, 000, 7, 000-8, 000 and less than 5, 000. In the kidney, 65Zn combined with three constituents of a molecular weight of more than 80, 000, 10, 000-20, 000 and less than 5, 000.