The hydrogenase content of the genome of Geobacter sulfurreducens, a member of the family Geobacteraceae within the delta-subdivision of the Proteobacteria, was examined and found to be distinct from that of Desulfovibrio species, another family of delta-Proteobacteria on which extensive research concerning hydrogen metabolism has been conducted. Four [NiFe]-hydrogenases are encoded in the G. sulfurreducens genome: two periplasmically oriented, membrane-bound hydrogenases, Hya and Hyb, and two cytoplasmic hydrogenases, Mvh and Hox. None of these [NiFe]-hydrogenases has a counterpart in Desulfovibrio species. Furthermore, the large and small subunits of Mvh and Hox appear to be related to archaeal and cyanobacterial hydrogenases, respectively. Clusters encoding [Fe]-hydrogenases and periplasmic [NiFeSe]-hydrogenases, which are commonly found in the genomes of Desulfovibrio species, are not present in the genome of G. sulfurreducens. Hydrogen-evolving Ech hydrogenases, which are present in the genomes of at least two Desulfovibrio species, were also absent from the G. sulfurreducens genome, despite the fact that G. sulfurreducens is capable of hydrogen production. Instead, the G. sulfurreducens genome contained a cluster encoding a multimeric Ech hydrogenase related (Ehr) complex that was similar in content to operons encoding Ech hydrogenases, but did not appear to encode a hydrogenase. Phylogenetic analysis revealed that the G. sulfurreducens ehr cluster is part of a family of related clusters found in both the Archaea and Bacteria.