Speciation of uranium in cultures of Clostridium sp. by X-ray absorption near-edge spectroscopy (XANES) at the National Synchrotron Light Source and by X-ray pho-toelectron spectroscopy (XPS) showed that U (VI) was reduced toU (IV). In addition toU (IV), a lower oxidation state of uranium, most probably U (III), was detected by XANES inthe bacterial cultures. Reduction of uranium occurred only in the presence of growing or resting cells. Organic acid metabolites, the extracellular components of the culture medium, and heat-killed cells failed to reduce uranium under anaerobic conditions. The addition of uranyl acetate or uranyl nitrate (> 210 µ U) to the culture medium retarded the growth of the bacteria as evidenced by an increase in the lag period before resumption of growth, by decreases in turbidity, and in the total production of gas and organic acid metabolites. These results show that uranium in wastes can be stabilized by the action of anaerobic bacteria.