The cytochrome c of Tetrahymena pyriformis GL (Phenoset A) had an isoelectric point of 6.5 and by sequence the following composition: Asp(7) Asn(2) Thr(4) Ser(8) Glu(6) Gln(2) Pro(7) Gly(13) Ala(13) Val(7) Met(2) Ile(5) Leu(6) Tyr(2) Phe(5) Lys(11) His(3) Trp(1) Arg(3) Cys(2) (total 109 residues). The peptides derived from the protein afforded complete overlap, so a complete sequence could be determined without reference of homologous proteins. Alignment with other mitochondrial cytochromes c required two internal deletions totalling three residues and an N-terminal region two residues longer than, and a C-terminal region one residue shorter than, the previously known limits. The sequence was the most divergent of the known mitochondrial cytochromes c, suggesting a distant relationship of ciliates to other eukaryotes. Details of the sequence data have been deposited as Supplementary Publication no. SUP 50068 (37 pages) at The British Library Lending Division, Boston Spa, Wetherby, West Yorkshire LS23 7 BQ, U.K., from whom copies can be obtained on the terms given in Biochem. J. (1976) 153,5.