ABSTRACTThe abundance ofGeobacterspecies in contaminated aquifers in which benzene is anaerobically degraded has led to the suggestion that someGeobacterspecies might be capable of anaerobic benzene degradation, but this has never been documented. A strain ofGeobacter, designated strain Ben, was isolated from sediments from the Fe(III)-reducing zone of a petroleum-contaminated aquifer in which there was significant capacity for anaerobic benzene oxidation. Strain Ben grew in a medium with benzene as the sole electron donor and Fe(III) oxide as the sole electron acceptor. Furthermore, additional evaluation ofGeobacter metallireducensdemonstrated that it could also grow in benzene-Fe(III) medium. In both strain Ben andG. metallireducensthe stoichiometry of benzene metabolism and Fe(III) reduction was consistent with the oxidation of benzene to carbon dioxide with Fe(III) serving as the sole electron acceptor. With benzene as the electron donor, and Fe(III) oxide (strain Ben) or Fe(III) citrate (G. metallireducens) as the electron acceptor, the cell yields of strain Ben andG. metallireducenswere 3.2 × 109and 8.4 × 109cells/mmol of Fe(III) reduced, respectively. Strain Ben also oxidized benzene with anthraquinone-2,6-disulfonate (AQDS) as the sole electron acceptor with cell yields of 5.9 × 109cells/mmol of AQDS reduced. Strain Ben serves as model organism for the study of anaerobic benzene metabolism in petroleum-contaminated aquifers, andG. metallireducensis the first anaerobic benzene-degrading organism that can be genetically manipulated.