An amino acid sequence is proposed for the cytochrome c4 from the bacterium Azotobacter vinelandii strain OP. It is a single polypeptide chain of 190 residues, with two sets of haem-attachment cysteine residues at positions 14/17 and 119/122. Proteins with similar sequences are also present in denitrifying pseudomonads. There is similarity in sequence between the two halves of the cytochrome c4 molecule, and each half also shows similarity to the sequences of certain monohaem cytochromes c isolated from organisms that are not obviously closely related to A. vinelandii. Detailed evidence for the amino acid sequence of the protein has been deposited as Supplementary Publication SUP 50125 (17 pages) at the British Library Lending Division, Boston Spa, West Yorkshire LS23 7BQ, U.K., from whom copies are available on prepayment.