The biosorption of thorium and uranyl ions by cells of Mycobacterium smegmatis has been studied as a function of initial cation concentration. A similar sorption saturation level was observed for both ions, For immobilized cells, optimal conditions of metal ion retention were found for a bacterial mass/support concentration ratio of 1/6, However, selective uptake of thorium was manifest in solutions of the mixed cations. X-ray diffraction studies of the heat-dried biomasses loaded with cations showed that uranyl-loaded samples present a distinct pattern typical of ammonium uranyl phosphate, whereas thorium-loaded samples are amorphous, The microorganism used appears to have useful properties for applications in connection with separation and concentration of natural radioelements under conditions of high dilution.