The amino acid sequence of rattlesnake cytochrome c was originally reported in 1965, and was one of the earlier sequences to be studied. When compared with other mitochondrial cytochromes c, the snake sequence was soon seen to be anomalous. There were several positions in which the snake protein resembled human cytochrome c, although comparable anomalies were not reported for the protein from other reptiles such as lizard and turtle. Explanations of these results have included accelerated evolution in the snake lineage, paralogy rather than orthology, and faulty determination of the sequence, and the rattlesnake is now often omitted from cytochrome c phylogenetic trees. We have re-investigated the sequence of the snake protein, and believe that the correct sequence differs in nine places from that used for evolutionary theorizing since 1965. Four of these differences are near the haem-attachment site, in a region that was only analysed for amino acid composition in the original investigation. The other five differences are towards the C-terminus of the molecule, and can be explained as being due to the wrong ordering of amino acids within peptides that had been satisfactorily purified. Despite these corrections, the rattlesnake cytochrome c sequence still more closely resembles human cytochrome c than it does that of any other protein we know. We believe that this is an example of convergent evolution, although it does appear that there has been accelerated change in the line connecting the rattlesnake to the ancestral vertebrate line. Detailed evidence for the amino acid sequence of the protein has been deposited as Supplementary Publication SUP 50162 (16 pages) at the British Library Document Supply Centre, Boston Spa. Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1991) 273, 5.