Bioremediation of chromium through the reduction of hexavalent chromium (as the chromate ion, CrO42-) is based on the notion that the product, trivalent chromium (Cr(III)), is less toxic than chromate. In this study, we show that soluble Cr(III), present at pH 6-8 as the Cr3+ ion and/or hydroxyl complexes (henceforth referred to as uncomplexed Cr(III)), can be found transiently in significant concentrations and has a deleterious effect on Shewanella sp. MR-4. However, Cr(lll) complexed to an organic ligand or precipitated as Cr(OH)3(s) has little or no effect on cells. Similarly, during the reduction of Cr(VI) by strain MR-4, complexation of the product, Cr(lll), results in increased cell survival and extended Cr(VI) reduction activity. These results and gene expression data obtained by qRT-PCR (quantitative reverse transcription-PCR) suggestthatthe observed toxic effect of Cr(II) formed during Cr(VI) reduction or added as an uncomplexed species is due to the interference with basic cell activities such as DNA transcription and/or replication. Important implications for the bioremediation of Cr(VI)-contaminated sites emerge from this study: Cr(VI) reduction by Shewanella sp. MR-4 is enhanced and sustained by the presence of compounds able to complex Cr(III) as it is being formed but, in turn, the complexation of Cr(III) precludes its precipitation and immobilization.