Variant Sedum alfredii Hance (V S. alfredii) could simultaneously take up U and Th from water with the highest concentrations recorded as 1.84 x 10(4) and 6.72 x 10(3) mg/kg in the roots, respectively. Th stimulated U uptake by V S. alfredii roots at Th-10 (10 mu M of Th), however, the opposite was observed at Th-100 (100 mu M of Th). A similar result was found in the effect of U on the uptake of Th by V S. alfredii. Subcellular fractionation studies of V S. alfredii indicated that U and Th were mainly stored in cell wall fraction, and much less was found in organelle and soluble fractions. Chemical form examination results showed that water-soluble U and Th were the predominant chemical forms in this plant. Addition of the other radionuclide in aqueous solutions altered the concentration and percentage of U or Th in cell wall fraction and in water-soluble form, resulting in the change of the uptake capacity of U or Th by V S. alfredii roots. Comparing with single U or Th treatment, the plant cells revealed more swollen chloroplasts and enhanced thickening in cell walls under the U-100 + Th-100 treatment, as observed by TEM. Those results collectively displayed that V S. alfredii may be utilized as a potential plant to simultaneously remove U and Th from aqueous solutions (rhizofiltration).