Several interesting amino acids (1) (2) (3) as well as toxic peptides (4) are known from fruit bodies of some species of Amanita. Among others a chlorine-containing amino acid, trans-2-amino-5-chloro-4-hexenoic acid (3), was recently isolated. We now report another chloro amino acid, L-2-amino-4-chloro-4-pentenoic acid, from a fungus of the same genus, Amanita pseudoporphyria Hongo. This amino acid has not yet been reported as a natural product. The new amino acid was isolated from the fruit bodies, purified, and its structure was determined by elementary analysis (consistent with C₅H₈NO₂Cl), optical rotation (indicating L-series), NMR, MS, catalytic hydrogenation to norvaline hydrochloride (identified by cellulose TLC and IR spectrum), oxidation with KMnO₄ to aspartic acid, and comparison with synthetic DL-2-amino-4-chloro-4-pentenoic acid (showing the same Rf-values on cellulose TLC with several different solvents, fragmentation patterns in MS and NMR spectra).