The adsorption of uranyl ion by microorganisms was examined. Among the 76 strains of 69 species tested (23 bacteria, 20 actinomycetes, 18 fungi, and 15 yeasts), high uranyl ion adsorption ability was exhibited by strains of the bacteria, Arthrobacter nicotianae, Bacillus subtilis, and Micrococcus luteus. A. nicotianae cells, which showed the best performance, could adsorb about 698 mg uranyl ion (2.58 mmol) per gram dry wt. of microbial cells. The adsorption of uranyl ion was rapid, selective, and mostly dependent on physico-chemical binding to the cell components. As well as uranyl ion, A. nicotianae could adsorb thorium ion with high efficiency. Cells immobilized with polyacrylamide gel could be used during repeated adsorption-desorption cycles.