The complete amino acid sequence of the major component myoglobin from finback whale, Balaenoptera physalus, was determined by the automated Edman degradation of several large peptides obtained by specific cleavages of the protein. Three easily separable peptides were obtained by cleaving with cyanogen bromide at the two methionine residues and one large peptide was isolated after cleavage with (2-p-nitrophenylsulfenyl)-3-methyl-3'-bromoindolenine. More than 60% of the covalent structure was established by the sequential degradation of three of these peptides and the apomyoglobin. An additional 30% of the primary sequence was established with peptides obtained from tryptic digestion of both the apomyoglobin and the acetimidoapomyoglobin, and the final 10% of the sequence was completed after digestion of the two larger cyanogen bromide peptides with S. aureus strain V8 protease. This myoglobin differs from that of the sperm whale, Physeter catodon, at 15 positions, from that of the arctic minke whale, Balaenoptera acutorostrata, at 3 positions, and from that of the California gray whale, Eschrichtius gibbosus, at 4 positions. All of the substitutions observed in this sequence fit easily into the three-dimensional structure of the sperm whale myoglobin.