Uptake of radioactive cobalt (58Co) by shellfish was determined by tracer experiments using mussel, Mytilus edulis, which has a world-wide distribution and is considered to be a sufficiently reliable indicator organism of radioactive contamination of the marine environment. As for soft tissues accumulation was high in the alimentary tract, and low in the mantle and the adductor muscle. However high concentration factors in the byssus (1, 500) and shell (190) were noticeable. By means of immersion experiments of shell pieces and cut off pieces of the byssus in 58Co containing sea water, it became evident that the high accumulation of 58Co by the byssus and shell was due to surface adsorption. Discussions are given on the utility of mussel as an indicator organism and on the role of structural protein of shellfish in the adsorption of radionuclides.