A biological process for the removal of heavy metals from the aqueous flows is described. Metals are precipitated on the surface of immobilized cells of a Citrobacter sp. as cell-bound metal phosphates. This uses phosphate liberated by the activity of a cell-bound phosphatase. Some radionuclides (e.g. 241americium) form metal phosphates readily; efficient removal of 241Am on a continuous basis is demonstrated. At low phosphatase activities, the efficiency of uranium removal correlates with enzyme activity. High phosphatase activities are not realised as an increase in metal removal, suggesting that chemical events become rate-limiting. Studies have suggested that maximal metal uptake occurs only after nucleation and the formation of precipitation foci. A model is presented to illustrate how nucleation and crystallization processes could enhance the removal of plutonium and neptunium from dilute solutions.