Cr(VI) reduction was observed during tests with Shewanella oneidensis MR-1 (previously named S. putrefaciens MR-1) while being grown with nitrate or fumarate as electron acceptor and lactate as electron donor. From the onset of anoxic growth on fumarate, we measured a gradual and progressive increase in the specific Cr(VI) reduction rate with incubation time until a maximum was reached at late exponential/early stationary phase. Under denitrifying conditions, the specific Cr(VI) reduction rate was inhibited by nitrite, which is produced during nitrate reduction. However, once nitrite was consumed, the specific reduction rate increased until a maximum was reached, again during the late exponential/early stationary phase. Thus, under both fumarate- and nitrate-reducing conditions, an increase in the specific Cr(VI) reduction rate was observed as the microorganisms transition from oxic to anoxic growth conditions, presumably as a result of induction of enzyme systems capable of reducing Cr(VI). Although Cr(VI) reduction has been studied in MR-1 and in other facultative bacteria under both oxic and anoxic conditions, a transition in specific reduction rates based on physiological conditions during growth is a novel finding. Such physiological responses provide information required for optimizing the operation of in situ systems for remediating groundwater contaminated with heavy metals and radionuclides, especially those that are characterized by temporal variations in oxygen content. Moreover, such information may point the way to a better understanding of the cellular processes used by soil bacteria to accomplish Cr(VI) reduction.