The gene encoding component H of the DNA-dependent RNA polymerase (RNAP, EC 126.96.36.199) of Sulfolobus acidocaldarius has been identified by comparison of the amino acid sequence with the derived amino acid sequence of an open reading frame (ORF88) in the RNAP operon. Corresponding genes were identified in Halobacterium halobium and were cloned and sequenced from Thermococcus celer and Methanococcus vannielii. All these rpoH genes are situated between the promoters of the RNAP operons and the corresponding rpoB and rpoB2 genes. The archaeal H subunits show high sequence similarity to each other and to the C-terminal portions of the largest of four subunits shared by all three specialized nuclear RNAPs. These correlations are further evidence for the striking similarity between archaeal and eucaryal RNAP structures and transcription systems.