Archaebacteria have been defined as a 'third primary kingdom' of cells in addition to the urkaryotes and the eubacteria. While the latter two correspond approximately to the conventional categories eukaryotes and prokaryotes respectively, the Archaebacteria have up to now comprised four groups of microorganisms: the methanogenic bacteria, the extremely halophilic bacteria and the two thermoacidophilic genera Sulfolobus and Thermoplasma. Based on ribosomal RNA sequence homologies and lipid composition, they apparently form a distinct group. Furthermore they possess or lack typical biochemical markers of both the eukaryotes and the prokaryotes, as well as having unique properties not found elsewhere. Altogether, this indicates that they are not closer to either one of the classical categories. One clear-cut difference between prokaryotes and eukaryotes is the diphtheria toxin reaction, which catalyses the covalent binding of adenosine diphosphate-ribose (ADPR) to the eukaryotic peptide elongation factor EF2 in contrast to the homologous prokaryotic factor EF-G. We report here that diphtheria toxin also catalyses the ADP-riboslation of archaebacterial elongation factors. In this respect, these factors have to be assigned to the EF2 type; we suppose that the ADP-ribosylatable structure arising so early in evolution is of fundamental importance for the elongation process.