A c‐type monohemic ferricytochrome c552 (11 kDa) was isolated from the soluble extract of a marine denitrifier, Pseudomonas nautica strain 617, grown under anaerobic conditions with nitrate as final electron acceptor. The NH2‐terminal sequence and the amino acid composition of the cytochrome were determined. The heme iron of the cytochrome c552 has histidine‐methionine as axial ligands, and a pH‐dependent mid‐point redox potential, equal to 250 mV at pH 7.6. The presence of methionine was demonstrated by visible, EPR and NMR spectroscopies. The assignment of most of the hemic protons was performed applying two‐dimensional NOE spectroscopy (NOESY), and the aromatic region was assigned through two‐dimensional correlated spectroscopy (COSY) experiments. The EPR spectrum of the oxidised form of the cytochrome c552 is typical of a low‐spin ferric heme.