The electron transfer pathway of Geobacter sulfurreducens has been intensively studied because of its ability of electron transfer to extracellular electron acceptors, such as Fe(III) and on electrode. However, the absence of overexpression system of G. sulfurreducens is one of the main obstacles for studying the physiology of G. sulfurreducens. OmcZ, an outer membrane-related c-type cytochrome of G. sulfurrducens, was homologously overexpressed via genomic integration. Quantitative RT-PCR analysis showed that the omcZ transcript in the knock-in strain was sixfold more abundant than in the wild type. Notably, omcZ expression appears to downregulate the expression level of OmcS, another outer membranerelated c-type cytochrome of G. sulfurreducens, based on the comparisons of total protein and transcript levels. This is the first report of the successful genetic overexpression system for studying functional genomics of G. sulfurreducens.