The complete amino acid sequence of the major component myoglobin from the dwarf sperm whale, Kogia simus, was determined by specific cleavage of the protein to obtain large peptides which are readily degraded by the automatic sequenator. Three easily separable peptides were obtained by cleaving the protein at its two methionine residues, and five peptides were obtained from the methyl acetimidated protein by cleavage with trypsin at the four arginine residues. Sequenator analysis of these fragments and the apomyoglobin provided over 80% of the covalent structure of the protein. The remainder of the primary structure was determined by further digestion of the two larger cyanogen bromide fragments with trypsin and staphylococcal protease. To reconfirm many of the substitutions found in this protein, the apomyoglobin was treated with 1,2-cyclohexanedione, and the resulting arginine protected protein was cleaved at its lysine residues with trypsin. This myoglobin differs from that of the sperm whale at 6 positions, and from the other cetacean myoglobins at about 16 positions. The appearance of a histidine residue at position 35 has no precedent in any myoglobin. The substitutions seen at positions 21, 51, and 132 are unique to date for cetacean myoglobins.