AbstractBiosorption of elements is a little understood phenomenon exhibited by some types of even nonliving microbial biomass. A common fungus Rhizopus arrhizus has been reported to take up uranium from aqueous solutions to the extent of 180 mg U6+/g. The mechanism of uranium sequestering by this type of biomass was studied by using experimental techniques such as electron microscopy, x‐ray energy dispersion analysis, IR spectroscopy, and supporting evidence was obtained for a biosorption mechanism consisting of at least three processes. Uranium coordination and adsorption in the cell‐wall chitin structure occur simultaneously and rapidly whereas precipitation of uranylhydroxide within the chitin microcrystalline cell‐wall structure takes place at a lower rate. Interference of Fe2+ and Zn2+ coions with uranium biosorption is indicated.