Partial sequence of the 16S ribosomal RNA gene of an extremely high radiotolerant bacterium, Rubrobacter radiotolerans (reclassified from Arthrobacter radiotolerans by chemical characteristics) was determined by PCR-amplification from a small amount of heat-lysed biomass, followed by direct sequencing of the PCR product. The sequence was aligned with seven species of Arthrobacter and also with representatives of various other bacterial groups. R. radiotolerans was confirmed to be out of the Arthrobacter group justifying the reclassification. Moreover, it has an individual position among the selected representatives being closer to the eubacterial groups.