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Despite the significant progress on iron reduction by thermophilic microorganisms, studies on their ability to reduce toxic metals are still limited, despite their common co-existence in high temperature environments (up to 70°C). In this study, Methanothermobacter thermautotrophicus , an obligate thermophilic methanogen, was used to reduce hexavalent chromium. Experiments were conducted in a growth medium with H 2 /CO 2 as substrate with various Cr 6+ concentrations (0.2, 0.4, 1, 3, and 5 mM) in the form of potassium dichromate (K 2 Cr 2 O 7 ). Time-course measurements of aqueous Cr 6+ concentrations with the 1, 5-diphenylcarbazide colorimetric method showed complete reduction of the 0.2 and 0.4 mM Cr 6+ solutions by this methanogen. However, much lower reduction extents of 43.6%, 13.0%, and 3.7% were observed at higher Cr 6+ concentrations of 1, 3 and 5 mM, respectively. These lower extents of bioreduction suggest a toxic effect of aqueous Cr 6+ to cells at this concentration range. At these higher Cr 6+ concentrations, methanogenesis was inhibited and cell growth was impaired as evidenced by decreased total cellular protein production and live/dead cell ratio. Likewise, Cr 6+ bioreduction rates decreased with increased initial concentrations of Cr 6+ from 13.3 to1.9 µM h -1 . X-ray absorption near-edge structure (XANES) spectroscopy revealed a progressive reduction of soluble Cr 6+ to insoluble Cr 3+ precipitates, which was confirmed as amorphous chromium hydroxide by X-ray diffraction and selected area electron diffraction pattern. However, a small fraction of reduced Cr occurred as aqueous Cr 3+ . Scanning and transmission electron microscope observations of M. thermautotrophicus cells after Cr 6+ exposure suggest both extra- and intracellular chromium reduction mechanisms. Results of this study demonstrate the ability of M. thermautotrophicus cells to reduce toxic Cr 6+ to less toxic Cr 3+ and its potential application in metal bioremediation, especially at high temperature subsurface radioactive waste disposal sites, where the temperature may reach ∼70°C.