We have isolated and sequenced the gene for a putative NADP-dependent glutamate dehydrogenase from the extremely halophilic archaebacterium Halobacterium salinarium. This gene is transcribed as a unique RNA molecule of about 1700 nucleotides. The 5end of the transcript contains characteristic consensus transcription initiation and promoter sequences observed in halophilic archaebacteria. The encoded polypeptide, with a predicted length of 435 amino acids, shows significant overall homology and conservation of functional domains when compared with different eubacterial and eukaryotic glutamate dehydrogenases. Surprisingly, the archaebacterial protein shares a larger number of identical amino acid residues with homologous polypeptides from higher eukaryotes than with those from unicellular eukaryotes and eubacteria.