Bonito cytochrome c was digested with trypsin, and the resulting peptides were fractionated by chromatography on a Dowex 1 column with volatile buffers. Heterogeneous peptide fractions were further resolved by Dowex 50 chromatography followed by paper electrophoresis or paper chromatography. The amino acid sequences of the tryptic peptides were determined mainly by the improved Edman method. Information on the arrangement of the tryptic peptides was obtained from the bridge peptides derived from the protein by treating with cyanogen bromide followed by chymotryptic digestion. The complete amino acid sequence of 103 residues was deduced. The number of total amino acid residues was less than that of other vertebrate cytochromes c (104 residues) with the exception of that of tuna cytochrome c. Only difference in the amino acid sequence between the tuna protein and the bonito protein was recognized at the 61st position.