The size, visible absorption spectra, nature of haem and haem content suggest that the cytochrome c peroxidase of Paracoccus denitrificans is related to that of Pseudomonas aeruginosa. However, the Paracoccus enzyme shows a preference for cytochrome c donors with a positively charged 'front surface' and in this respect resembles the cytochrome c peroxidase from Saccharomyces cerevisiae. Paracoccus cytochrome c-550 is the best electron donor tested and, in spite of an acidic isoelectric point, has a markedly asymmetric charge distribution with a strongly positive 'front face'. Mitochondrial cytochromes c have a much less pronounced charge asymmetry but are basic overall. This difference between cytochrome c-550 and mitochondrial cytochrome c may reflect subtle differences in their electron transport roles. A dendrogram of cytochrome c1 sequences shows that Rhodopseudomonas viridis is a closer relative of mitochondria than is Pa. denitrificans. Perhaps a mitochondrial-type cytochrome c peroxidase may be found in such an organism.