Shewanella oneidensis is a widely distributed species of bacteria and is known to utilize several elements such as iron, manganese and sulfur as electron acceptors. In an anoxic environment lacking more electrochemically favourable electron acceptors S. oneidensis is shown to reduce uranium, changing its oxidation state from hexavalent to tetravalent, by the following reaction: H2+ UO22+ → 2H+ + UO2. The uranyl solution concentration (U(VI)) was measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES), and the reduction data were fit to first order. Several cell concentrations were examined and both the rate of uranyl reduction and the total amount of uranyl reduced are found to be dependent upon cell density. The largest rate constant was 0.7 hr-1 corresponding to a cell density of 2.4*109 cells/mL and an initial reduction rate of 1414 μM/hr. A cell concentration of 6.6*108 cells/mL gave rise to an initial reduction rate of 400μM U(VI) per hour, and had, within a period of 72 hours approximately 98% of the original 2 mM uranyl acetate reduced, as opposed to only 87% for 2.4*109 cells/mL.