The amino acid composition of β-amylase [EC 3. 2.1. 2] from sweet potatoes grown in Japan was determined. The analyses indicate that the enzyme is composed of the following 419 amino acid residues per 50, 000 g of the enzyme: Asp54, Thr15, Ser18, Glu41, Pro24, Gly35, Ala34, Val31, Met13, Ile19, Leu32, Tyr18, Phe18, Lys23, His7, Arg17, Trp10, and CyS5. No disulfide bond is present in the enzyme. Discrepancies exist between the results reported here and those reported by Thoma et al. (7). All of sulfhydryl groups involved in the enzyme are relatively unreactive in the native protein. The terminal amino acid residues were investigated. One mole of NH2-terminal alanine per 50, 000 g of enzyme was found by the FDNB* method. The COOH-ter-minal amino acid was analyzed by hydrazinolysis and treatment with carboxypep-tidase A [EC 3. 4. 2. 1], However, the results were not consistent with each other, possibly because there may be a prolyl-glycine and/or -x-proline COOH-terminal sequence in the enzyme.