The amino acid sequences of the cytochromes c-554(548) and c' from the moderately halophilic bacterium Paracoccus sp., I.A.M. 203 (= A.T.C.C. 12084, N.C.I.B. 8669) have been determined. Cytochrome c-554(548) consists of a single polypeptide chain of 83 residues, and dimerizes strongly. The most similar protein of known sequence is the N-terminal half of the dihaem cytochrome c4, and other related proteins include the cytochrome c-554(547) of Thiobacillus neapolitanus and the cytochrome c-553 of Desulfovibrio vulgaris. Cytochrome c', which has a single polypeptide chain of 132 residues, is similar in sequence to cytochromes c' from phototrophic and denitrifying bacteria, but only shows about 36% sequence identity to the most similar protein of known sequence. Both of the Paracoccus proteins have a considerable excess of acidic amino acid side chains over basic ones, and a higher proportion of their basic amino acids is arginine than is usual in cytochromes c. Both these characteristics seem to be adaptations to increase the stability of the proteins in an environment of high ionic strength. Detailed evidence for the amino acid sequences of the proteins has been deposited as Supplementary Publication 50140 (24 pp.) at the British Library (Lending Division), Boston Spa, Yorkshire LS23 7BQ, U.K. from which copies are available on prepayment.