Shewanella oneidensis strain MR-1 is a dissimilatory metal-reducing bacterium frequently found in aquatic sediments. In the absence of oxygen, S. oneidensis can respire extracellular, insoluble oxidized metals, such as iron (hydr)oxides, making it intimately involved in environmental metal and nutrient cycling. The reduction of ferric iron (Fe3+) results in the production of ferrous iron (Fe2+) ions, which remain soluble under certain conditions and are toxic to cells at higher concentrations. We have identified an inner membrane protein in S. oneidensis, encoded by the gene SO_4475 and here called FeoE, which is important for survival during anaerobic iron respiration. FeoE, a member of the cation diffusion facilitator (CDF) protein family, functions to export excess Fe2+ from the MR-1 cytoplasm. Mutants lacking feoE exhibit an increased sensitivity to Fe2+. The export function of FeoE is specific for Fe2+, as an feoE mutant is equally sensitive to other metal ions known to be substrates of other CDF proteins (Cd2+, Co2+, Cu2+, Mn2+, Ni2+, or Zn2+). The substrate specificity of FeoE differs from that of FieF, the Escherichia coli homolog of FeoE, which has been reported to be a Cd2+/Zn2+ or Fe2+/Zn2+ exporter. A complemented feoE mutant has an increased growth rate in the presence of excess Fe2+ compared to that of the ΔfeoE mutant complemented with fieF. It is possible that FeoE has evolved to become an efficient and specific Fe2+ exporter in response to the high levels of iron often present in the types of environmental niches in which Shewanella species can be found.