Biomass from several fungal species removed thorium from solution in 1M HNO3, pH 0–1. Thorium uptake was saturable with increasing thorium concentration, although the equilibria did not correspond to a simple ad sorption isotherm. Thorium uptake was altered by the biomass concentration, the uptake per unit biomass being reduced at high biomass concentrations. The presence of Al3+ and Fe3+ only slightly inhibited uptake of thorium while Ca2+, Mg2+, and Na+ had no effect. Thus fungal biomass appears capable of removing thorium from solution under chemical conditions existing in acid waste liquors. Thorium uptake was increased by pretreatment using detergent and also, in the case of filamentous fungi, varied with the culture conditions, which implies that the thorium uptake characteristics of fungal biomass are able to be manipulated by these or similar means for optimum performance.