Thermoplasma acidophilum is a thermoacidophilic archaebacterium occupying a paradoxical place in phylogenetic trees (phenotypically it is a thermoacidophile but phylogenetically it classifies with the methanogens). To better understand its phylogeny, the pyruvate kinase from this organism is being investigated as a molecular marker. The enzyme has been purified and has a native Mr of 250 000. It consists of four, apparently identical subunits each of Mr 60 000. No remarkable kinetic differences have been found between this thermophilic enzyme and its mesophilic counterparts other than its greater thermostability. Its amino acid composition has been determined and some partial sequencing has been done.