Partial sequences were derived for the 16S-like ribosomal RNA of Halteria grandinella, Glaucoma chattoni, Colpidium campylum, Opisthonecta henneguyi and Colpoda inflata. Using isolated bulk nucleic acid preparations, partial rRNA sequences were determined using a series of oligonucleotide primers complementary to conserved sequence elements of the molecule and a dideoxynucleotide sequencing reaction using reverse transcriptase. Sequences were aligned and an evolutionary tree was generated by a distance-matrix method, comparing these partial sequences to the complete sequences of species of Tetrahymena, Paramecium, Euplotes, Oxytricha and Stylonychia. Halteria shows greater similarity to the stichotrichs Oxytricha and Stylonychia than to the hypotrich Euplotes, while these four genera share the same branch. Glaucoma and Colpidium appear closely related to Tetrahymena, with all three being more closely related to Opisthonecta than to Paramecium. Colpoda diverges near the base of the branch shared by Paramecium and the four oligohymenophorean genera. These results are discussed with reference to published revisions of the systematics of the phylum Ciliophora.