The nucleotide sequences of the 5S and 5.8S rRNAs of eight strains of tetrahymenine ciliates have been determined. The sequences indicate a clear distinction between Tetrahymena paravorax and its suggested conspecific T. vorax, but leave the taxonomic distinction between T. vorax and T. leucophrys in doubt. The rRNA sequences of six Tetrahymena species and of three other species of the suborder Tetrahymenina have been used to deduce evolutionary schemes in which ancestral rRNA sequences and changes are proposed. These schemes suggest the predominant acceptance of G----A and C----T transitions in the 5S rDNA during the evolution of the suborder.