Uptake of Zn⁶⁵ was followed in Microcystis littoralis, a common littoral bluegreen alga of Bombay and Chlorella vulgaris, a green alga for periods of fifteen days. Microcystis showed a maximum uptake value after five hours and there was an inverse relationship between the uptake and population growth patterns. Apparent equilibrium was reached in ten to fourteen days when the concentration factor was found to be 273. In Chlorella the uptake pattern followed closely that of the population growth and after about fourteen days the concentration of activity in the medium was considerably depleted. Zn⁶⁵ activity in the control (filtered seawater into which activity has been added) remained fairly constant in both cases indicating that the loss of activity due to adsorption on the sides of the container was negligible.