The crystal structure of methemerythrin from Themiste dyscritum has been determined at 2.8-Angstrom resolution by single isomorphous replacement technique combined with anomalous scattering from a K2HgI4 derivative. Noncrystallographic symmetry relating the four subunits in the asymmetric unit was used to obtain an average electron density map of the hemerythrin monomer, and a computer graphics system was used to fit a polypeptide model to the electron density. The average map was of sufficient quality to locate most of the amino acid side chains and to confirm the assignment of His-25, His-54, Glu-58, His-73, His-77, His-101, Asp-106, and Tyr-109 as the iron ligands. One of the mercury sites in the heavy atom derivative is located between two Cys-9 residues related by a noncrystallographic twofold axis, although no intersubunit disulfide bond is present in the native structure. The residues responsible for the binding of the subunits to form the octamer are identified.