Heterodisul®de reductase (Hdr) is a unique disul®de reduc-tase that plays a key role in the energy metabolism of methanogenic archaea. The genome of the sulfate-reducing archaeonArchaeoglobus fulgidusencodes several proteins of unknown function with high sequence similarity to the catalytic subunit of Hdr. Here we report on the puri®cation of a multisubunit membrane-bound enzyme complex from A.