A new chlorinated amino acid was isolated from Amanita vergineoides. Further survey of this mushroom led to the isolation of the new chlorinated amino acid, and in this paper the isolation and structural elucidation of this amino acid are reported. The stereostructure was elucidated as (S)-cis-2-amino-5-chloro-4-pentenoic acid on the basis of spectroscopic analysis (FD-MS, IR, NMR, CD) and coupling constant data for the cis double bond geometry, with the (S)-configuration confirmed by CD spectroscopy.