A ferredoxin was purified from extremely halophilic archaeon Haloarcula japonica strain TR-1, and then characterized. The Ha. japonica ferredoxin proved to contain a 2Fe-2S cluster. A part of the gene encoding the ferredoxin was amplified by PCR. Subsequently, the entire ferredoxin gene was cloned from chromosomal DNA of Ha. japonica using the PCR product as a probe. The structural gene consisted of an open reading frame of 387 nucleotides. The deduced amino acid sequence showed 89-98% identities with those of the ferredoxins from other extremely halophilic archaea.