Unrooted phylogenetic dendrograms were calculated by two independent methods, parsimony and distance matrix analysis, from an alignment of the derived amino acid sequences of the A and C subunits of the DNA-dependent RNA polymerases of the archaebacteria Sulfolobus acidocaldarius and Halobacterium halobium with 12 corresponding sequences including a further set of archaebacterial A + C subunits, eukaryotic nuclear RNA polymerases, pol I, pol II, and pol III, eubacterial β′ and chloroplast β′ and β″ subunits. They show the archaebacteria as a coherent group in close neighborhood of and sharing a bifurcation with eukaryotic pol II and (or) pol IIIA components. The most probable trees show pol IA branching off from the tree separately at a bifurcation with the eubacterial β′ lineage. The implications of these results, especially for understanding the possibly chimeric origin of the eukaryotic nuclear genome, are discussed.Key words: transcription, evolution, taxonomy, subunits, gene organization.