The soil bacterium Bradyrhizobium (Chamaecytisus) strain BGA-1 produces an extracellular polymeric substance (EPS) that, in the presence of Fe3+, Al3+ or Th4+ solutions, forms a gel-like precipitate composed of polysaccharide, protein, lipopolysaccharide and the metal. Precipitation of the main component of the EPS, the extracellular polysaccharide, and thorium was studied. The precipitate was stable, but redissolved at pH values below 3.0 or in the presence of 10 mM EDTA. In the precipitate, the ratio thorium/basic repeating unit of the polysaccharide ranged from 0.4 to 0.8 mol/mol. Soluble polysaccharide-thorium complexes were not found, and larger polysaccharide molecules were precipitated in preference to smaller ones. Kinetic studies showed a non-linear dependence of the precipitate on the concentrations of both thorium and polysaccharide. The behaviors of the purified polysaccharide and of whole EPS with the thorium-containing precipitate were compared. The results suggested that EPS components other than polysaccharide are able to modify the precipitating ability of the polysaccharide. Thus, whole EPS is a better substrate than the purified polysaccharide for the removal of thorium from its solutions.