Oligonucleotide catalogs have been determined for the 16S ribosomal RNAs of three sulfur dependent (i.e. "thermoacidophilic") archaebacteria--Sulfolobus acidocaldarius, S. solfataricus, and Thermoproteus tenax. The three form a group specifically related to one another, but are only distantly related to the other archaebacteria--i.e. the group comprising the methanogens, extreme halophiles, and (peripherially) the genus Thermoplasma. The three catalogs exhibit two features unique among bacteria: (1) an unusually high number of long pyrimidine runs, and (2) a remarkably high number of (post-transcriptionally) modified nucleotides.