Although most of the key components of the transcription apparatus, and in particular, RNA polymerase (RNAP) subunits, are conserved between archaea and eukaryotes, no archaeal homologs of the small RPB8 subunit of eukaryotic RNAP have been detected. We report that orthologs of RPB8 are encoded in all sequenced genomes of hyperthermophilic Crenarchaeota and a recently sequenced "korarchaeal" genome, but not in Euryarchaeota or the mesophilic crenarchaeon Cenarchaeum symbiosum. These findings suggest that all 12 core subunits of eukaryotic RNAPs were already present in the last common ancestor of the extant archaea.