A Citrobacter sp., documented for heavy metal uptake, removed La and U from a flow presented to polyacrylamide gel-immobilized cells but removed little Th under the same conditions when presented alone or in combination with La and U. The poor removal of Th was attributable to the strength of the complex of tetravalent actinide species with citrate cc-presented as a model chelating ligand. The implications of this for the treatment of wastes containing Pu(IV) are discussed.