The amino acid sequence of cytochrome c purified from rice, Oryza sativa L., was determined. The complete amino acid sequence of rice cytochrome c is as follows: Ac-Ala-8-Ser-Phe-Ser-Glu-Ala-Pro-Pro-Gly1-Asn-Pro-Lys-Ala-Gly-Glu-Lys-Ile-Phe10-Lys-Thr-Lys-Cys-Ala-Glx-Cys-His-Thr-Val20-Asp-Lys-Gly-Ala-Gly-His-Lys-Glx-Gly-Pro30-Asx-Leu-Asx-Gly-Leu-Phe-Gly-Arg-Glx-Ser40-Gly-Thr-Thr-Pro-Gly-Tyr-Ser-Tyr-Ser-Thr50-Ala-Asp-Lys-Asn-Met-Ala-Val-Ile-Trp-Glx60-Glx-Asx-Thr-Leu-Tyr-Asp-Tyr-Leu-Leu-Asn70-Pro-TML-Lys-Tyr-Ile-Pro-Gly-Thr-Lys-Met80-Val-Phe-Pro-Gly-Leu-TML-Lys-Pro-Glx-Glx90-Arg-Ala-Asp-Leu-Ile-Ser-Tyr-Leu-Lys-Glu100-Ala-Thr-Ser (Ac = acetyl group, TML = epsilon-N-trimethyllsine). The primary structure of rice cytochrome c was found to be homologous with those of other plant cytochromes c reported so far; it possesses general features common to plant cytochromes c, and all the invariant residues characterized in dicotyledonous cytochromes c are also conserved in the sequence of rice cytochrome c, as well as those of other monocotyledonous cytochromes c. The distinctive features of rice cytochrome c are a high content of proline residues, their unique locations in the sequence and the presence of a serine residue at position 96.