A laboratory experiment of accumulation ana excretion of radionuclides (125I, 57Co, 141Ce, 103Ru, 85Sr, 137Cs, 54Mn) in juvenile Japanese flounder was carried out in order to elucidate the genetic factor intervening in the mineral balance, which possibly caused the fluctuation of so-called concentration factors. Fish originating from two populations, namely clone brood and hatchery-reared brood, were used in the present study. The variance of concentration of radionuclides was compared in terms of the radionuclide activity ratio, which was defined by radionuclide concentration in fish normalized by that in surrounding water. In general, narrower variations of concentration were observed in clone brood than in hatchery-reared brood, and significant differences were observed for57Co, 141Ceand137Csin the accumulation process and for five radionuclides other than125Iand103Ruin excretion process. The present study suggested that a probability of application of clonal fish would be advantageous in experimental assessments of biological effects of environmental contaminants in the sea.