Although the structure of the catalytic RNA component of ribonuclease P has been well characterized in Bacteria, it has been little studied in other organisms, such as the Archaea, We have determined the sequences encoding RNase P RNA in eight euryarchaeal species: Halococcus morrhuae, Natronobacterium gregoryi, Halobacterium cutirubrum, Halobacterium trapanicum, Methanobacterium thermoautotrophicum strains Delta H and Marburg, Methanothermus fervidus and Thermococcus celer strain AL-1, On the basis of these and previously available sequences from Sulfolobus acidocaldarius, Haloferax volcanii and Methanosarcina barkeri the secondary structure of RNase P RNA in Archaea has been analyzed by phylogenetic comparative analysis, The archaeal RNAs are similar in both primary and secondary structure to bacterial RNase P RNAs, but unlike their bacterial counterparts these archaeal RNase P RNAs are not by themselves catalytically proficient in vitro.