The gene encoding the [Fe]-hydrogenase from the anaerobic bacterium Megasphaera elsdenii has been cloned and sequenced. The gene is monocistronic, in keeping with the protein being a monomer. The translated protein sequence (484 residues, M(r)=53 kDa) comprises a small 2[4Fe-4S] ferredoxin-like domain and a large domain containing the catalytic site. Comparisons with other [Fe]-hydrogenase sequences, including two of which the crystal structures are known, show that the M. elsdenii protein is among the smallest of these enzymes and provide useful indications regarding the basic structural core common to all [Fe]-hydrogenases. It is, nevertheless, to be noted that the genome of Thermotoga maritima encodes a putative [Fe]-hydrogenase that would consist of only 301 residues.