The neurotoxic amino acids, acromelic acids A and B were efficiently isolated from the poisonous mushroom Clitocybe acromelalga , accompanied by a new toxic analogue, acromelic acid C. The structure of acromelic acid C was elucidated to be 1 by comparison of the spectral data with those of acromelic acids A and B. Acromelic acid C exhibited a lethal toxicity to the mice at a dose of 10 mglkg.