The oral filaments of the ciliate Tetrahymena consist of the tetrins, insoluble polypeptides with molecular masses of around 85 kD. We characterised the tetrins of T. thermophila by two-dimensional gels and derived a large number of peptide sequences by in gel digestion. Using RT-PCR techniques and RACE-PCR, the complete cDNA sequences of tetrins A, B and C were established. Although tetrins differ strikingly in protein sequence they show a common structural principle. A N-terminal domain of 60 to 100 residues contains most of the proline residues of the tetrins and is probably globular. It is followed by a long alpha-helical domain of 620 to 640 residues which either lacks prolines or in tetrin A contains a single proline residue. Although this long domain has coiled coil forming ability, the individual heptad repeats are not extensive. Tetrins are novel cytoskeletal proteins unique to ciliates. Since the three tetrin sequences account for all 900 amino acid residues obtained by microsequencing of peptides, an additional major tetrin seems excluded. A minor component D is related to tetrin B by peptide sequences. The isoelectric variants, particularly obvious for tetrin A, most likely reflect post-translational modifications. These could arise by phosphorylation of serines and threonines in the proline rich N-terminal domain.