A peptide which inhibits the angiotensin-converting enzyme (ACE) was separated from sequential hydrolysates of defatted Antarctic krill muscle by pepsin and trypsin. The preparation procedure included chromatography on SP-Sephadex C-25, Superose 12, and reverse phase HPLC. The peptide fraction with the ACE-inhibiting activity was nearly pure and the main component was found to be a peptide with the amino acid sequence of Lys-Leu-Lys-Phe-Val showing a half-maximum inhibition concentration (1C50) of 30 μmol/l. A peptide sequence with 66% homology to the present peptide was found in some proteins such as prostaglandin DII reductase, thrombospondin precursor, epidermal growth factor precursor.