The complete amino acid sequence of muscle hemerythrin (myohemerythrin) from the sipunculid Themiste (syn. Dendrostomum) pyroides has been determined by analysis of tryptic, chymotryptic, and cyanogen bromide peptides. The primary structure of myohemerythrin differs substantially from that of coelomic hemerythrins of Phascolopsis (syn. Golfingia) gouldii and Themiste pyroides, the amino acid sequence of the muscle protein being only 46 and 45% homologous with the respective coelomic hemerythrins. The most extensive regions of homology between muscle and coelomic proteins occur near the terminii. These and other shorter regions of homology are interpreted in terms of the essential iron ligand residues of the active center.