A strongly acidic amino acid-N-carboxymethyl-L-serine-, not previously known in nature, has been isolated from asparagus (Asparagus officinalis) shoots. Some unique properties of this amino acid, such as a much bigger mobility to anode on high voltage paper electrophoresis (pH 3.6) than aspartic acid and characteristic changes of NMR spectra in aqueous solution with various pD, were discussed in relation to its structure. Six acidic dipeptides and four cysteine derivatives have been isolated from the acidic amino acids fraction of asparagus (Asparagus officinalis) shoots.1} This paper reports the isolation of strongly acidic amino acid-N-carboxymethyl-L-serine (CMS)-, hitherto unknown in nature, from asparagus shoots. The facts that CMS was isolated from the effluent out of Amberlite IR-120 (H+), that CMS moves far greater distance to an anode than aspartic acid on high voltage paper electrophoresis (HVE) (pH 3.6) and that both of methylene protons, -CH2-COOH, and methine proton, -CH-COOH, show a big down-field shift on NMR spectra when aqueous solution of CMS was acidified to form a cationic form are discussed in relation to the structure of CMS.