The amino acid sequence of locust cytochrome c was determined, although the overlap between chymotryptic and tryptic peptides at residues tyrosine-97 and leucine-98 was not observed, owing to an anomalous tryptic break duplicating the chymotryptic digestion. The molecule consists of a single polypeptide chain of 107 residues, homologous with other mitochondrial cytochromes c. In common with other known insect cytochromes c, it possesses a non-acetylated, four-residue tail at the N-terminus relative to glycine-1 of the standard alignment. A molecular phylogeny for 17 species was constructed relating the cytochrome c molecules of Schistocerca gregaria and other invertebrates with those of representative taxonomic groups. Experimental details are given in a supplementary paper deposited as Supplementary Publication SUP 50077 (24 pages) at the British Library Lending Division, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can obtained on the terms indicated in Biochem. J. (1977) 161, 1.