Four kinds of bioreactor were evaluated for thorium removal by fungal biomass. Static-bed or stirred-bed bioreactors did not give satisfactory thorium removal probably because of poor mixing. An air-lift bioreactor removed approximately 90-95% of the thorium supplied over extended time periods and exhibited a well-defined breakthrough point after biosorbent saturation. The air-lift bioreactor promoted efficient circulation and effective contact between the thorium solution and the mycelial pellets. Of several fungal species tested, Rhizopus arrhizus and Aspergillus niger were the most effective biosorbents with loading capacities of 0.5 and 0.6 mmol g-1 respectively (116 and 138 mg g-1) at an inflow thorium concentration of 3 mmol dm-3. The efficiency of thorium biosorption by A. niger was markedly reduced in the presence of other inorganic solutes while thorium biosorption by R. arrhizus was relatively unaffected. Air-lift bioreactors containing R. arrhizus biomass could effectively remove thorium from acidic solution (1 mol dm-3 HNO3) over a wide range of initial thorium concentrations (0.1-3 mmol dm-3). The biotechnological application and significance of these results are discussed in the wider context of fungal biosorption of radionuclides.