The primary structure of the gene for the elongation factor EF-Tu from the halophilic archaebacterium Halobacterium marismortui (hEF-Tu) is described. It is the first gene of a halophilic elongation factor EF-Tu to be sequenced. When the sequence of hEF-Tu is compared to that of homologous proteins from other organisms, the highest identity (61%) is found with EF-Tu from Methanococcus vannielii, a non-halophilic archaebacterium. In the search for halophilic characteristics therefore the most appropriate comparison is with the M. vannielii sequence. The excess of acidic amino acid residues in the hEF-Tu sequence (already observed in the composition of other halophilic proteins) results to a large extent from changes of Lys, Asn or Gin to Asp or Glu. A structural analysis algorithm applied to the halophilic sequence places these acidic residues on the surface of the protein. The corresponding residues in the crystal structure of the first domain of EF-Tu from E. coli (the only EF-Tu structure available) are grouped in patches on the protein surface, in each of which several residues that may be far apart in the sequence come quite close to each other in the tertiary structure.