L-Malate dehydrogenase from the extremely thermophilic mathanogen Methanothermus fervidus was isolated and its phenotypic properties were characterized. The primary structure of the protein was deducted from the coding gene. The enzyme is a homomeric dimer with a molecular mass of 70 kDa, possesses low specificity for NAD+ or NADP+ and catalyzes preferentially the reduction of oxalacetate. The temperature dependence of the activity as depicted in the Arrhenius and van't Hoff plots shows discontinuities near 52 degrees C, as was found for glyceraldehyde-3-phosphate dehydrogenase from the same organism. With respect to the primary structure, the archaebacterial L-malate dehydrogenase deviates strikingly from the eubacterial and eukaryotic enzymes. The sequence similarity is even lower than that between the L-malate dehydrogenases and L-lactate dehydrogenases of eubacteria and eukaryotes. The phylogenetic meaning of this relationship is discussed.