In situ bioreduction of soluble hexavalent uranium U(VI) to insoluble U(IV) (as UO2) has been proposed as a means of preventing U migration in the groundwater. This work focuses on the bioreduction of U(VI) and precipitation of U(IV). It uses anaerobic batch reactors with Desulfovibrio vulgaris, a well-known sulfate, iron, and U(VI) reducer, growing on lactate as the electron donor, in the absence of sulfate, and with a 30-mM bicarbonate buffering. In the absence of sulfate, D. vulgaris reduced >90% of the total soluble U(VI) (1mM) to form U(IV) solids that were characterized by X-ray diffraction and confirmed to be nano-crystalline uraninite with crystallite size 2.8 +/- 0.2nm. pH values between 6 and 10 had minimal impact on bacterial growth and end-product distribution, supporting that the mono-nuclear, and poly-nuclear forms of U(VI) were equally bioavailable as electron acceptors. Electron balances support that H-2 transiently accumulated, but was ultimately oxidized via U(VI) respiration. Thus, D. vulgaris utilized H-2 as the electron carrier to drive respiration of U(VI). Rapid lactate utilization and biomass growth occurred only when U(VI) respiration began to draw down the sink of H-2 and relieve thermodynamic inhibition of fermentation.