The uptake, accumulation and loss of traces of radiocobalt present in the culture medium by a green alga, Chlorella vulgaris, were studied. The trace quantities of the radionuclide employed had no effect on the growth of the algal population. The uptake is related to the number of cells present and to the temperature of the culture medium; it is not affected by mitotic activity. The rate of the 60Co, uptake during algal growth is compared to the rate of uptake of other radioisotopes; 90Sr and, to some extent, 137Cs have the same rate of uptake as 60Co, whereas 90Y and 144Ce differ. Perhaps the radiocobalt in the cells undergoes a chelating process and consequently its loss to the outer environment is very slow. The involvement of a biological process, in addition to a physical one, is claimed in order to explain the possibility of an active concentration of the radioisotope (C.F.=170). Emphasis is laid on the possible danger of this concentration when a disposal system by dilution of industrial or nuclear waste is employed.