A new acidic amino acid was isolated from a Basidiomycetes, Lactarius piperatus (Fr.) S. F. GRAY. The structure of this compound has been determined as 2(S), 3'(S)-1-(3-amino-3-carboxypropyl)-5-oxo-2-pyrrolidinecarboxylic acid (1) on the basis of nuclear magnetic resonance analysis and chemical synthesis.