The complete amino acid sequence of glutamate dehydrogenase from the thermoacidophilic archaebacterium Sulfolobus solfataricus has been determined. The sequence was reconstructed by automated sequence analysis of peptides obtained after cleavage by trypsin, cyanogen bromide, Staphylococcus aureus V8 protease and pepsin. The enzyme subunit is composed of 421 amino acid residues yielding a molecular mass of 46.078 kDa. The presence of N-epsilon-methyllysine in six positions of the sequence was observed. Comparison of the sequence of glutamate dehydrogenase from S. solfataricus with the other known primary structures of the corresponding enzyme from different sources, gives an overall identity of 9.2% and shows a symmetrical evolutionary distance of this archaebacterial protein from the two groups of vertebrate on one side and eubacterial and low eucaryote enzymes on the other side. The occurrence of specific substitutions and a possible role for N-epsilon-methylation of lysine residues are discussed in view of current hypotheses on the molecular basis of thermal adaptation of proteins.