A cytochrome c-550, with mid-point potential +265 mV, has been purified from Thiosphaera pantotropha. The cytochrome was recognised by antibodies to Paracoccus denitrificans cytochrome c-550, but the two proteins were not immunologically identical. Amino acid sequencing of the cytochrome c-550 showed 85.9% and 95.5% identities, respectively, with the cytochromes c-550 of P. denitrificans and Thiobacillus versutus; these are amongst the highest values reported for similarities between class I c-type cytochromes of the c2 group. These similarities are consistent with the published values of 85% for the overall DNA similarity of P. denitrificans and T. pantotropha, but contrast with published 16S rRNA analyses which indicate identity between T. pantotropha and P. denitrificans and 97.5% similarity of T. versutus with these two organisms. Analysis by plasma-desorption mass spectrometry of the peptide containing the haem-binding motif isolated from the apocytochrome has shown that an Hg atom binds to one or both of the two thiol groups.