The formation of outer membrane (OM) cytochromes seems to be a key step in the evolution of dissimilatory iron-reducing bacteria. They are believed to be the endpoints of an extended respiratory chain to the surface of the cell that establishes the connection to insoluble electron acceptors such as iron or manganese oxides. The gammaproteobacterium Shewanella oneidensis MR-1 contains the genetic information for five putative OM cytochromes. In this study, the role and specificity of these proteins were investigated. All experiments were conducted using a markerless deletion mutant in all five OM cytochromes that was complemented via the expression of single, plasmid-encoded genes. MtrC and MtrF were shown to be potent reductases of chelated ferric iron, birnessite, and a carbon anode in a microbial fuel cell. OmcA-producing cells were unable to catalyze iron and electrode reduction, although the protein was correctly produced and oriented. However, OmcA production resulted in a higher birnessite reduction rate compared with the mutant. The presence of the decaheme cytochrome SO_2931 as well as the diheme cytochrome SO_1659 did not rescue the phenotype of the deletion mutant.