The sequence of the small-subunit rRNA from the thermoacidophilic archaebacterium Sulfolobus solfataricus has been determined and compared with its counterparts from halophilic and methanogenic archaebacteria, eukaryotes, and eubacteria. The S. solfataricus sequence is specifically related to those of the other archaebacteria, to the exclusion of the eukaryotic and eubacterial sequences, when examined either by evolutionary distance matrix analyses or by the criterion of minimum change (maximum parsimony). The archaebacterial 16S rRNA sequences all conform to a common secondary structure, with the S. solfataricus structure containing a higher proportion of canonical base pairs and fewer helical irregularities than the rRNAs from the mesophilic archaebacteria. S. solfataricus is unusual in that its 16S rRNA-23S rRNA intergenic spacer lacks a tRNA gene.