An odorless, and slightly acid and acrid component was extracted from Chondria armata (KÜTZING) OKAMURA (Rhodomelaceae) (Japanese name, “Hanayanagi” or “Domoi”) as colorless needle crystals, m.p. 217° (decomp.), [α]D12-109.6° (c=1.314, H2O), UV: λmaxH2O 242mμ (log ε 4.42), C15H21O6N⋅2H2O. Oral administration of 20mg. of this substance was found to be markedly effective in expelling ascaris and pinworm, without any observable side effects. This component was considered to be the anthelmintic principle of Chondria armata and was named “domoic acid.” The acid is soluble in water and acetic acid, easily soluble in dil. mineral acid and alkali solution, and dissolves in sodium carbonate and hydrogen carbonate solution with effervescence. It is sparingly soluble in methanol and ethanol, and insoluble in ether, petroleum ether, chloroform, benzene, and acetone. It discolors bromine water and potassium permanganate solution, indicating unsaturation. It colors yellow to ninhydrin, vanillin, and alloxane, red to Folin reagent, and indigo blue to Feigl's aliphatic secondary amine reaction. Domoic acid forms salts sparingly soluble in water with lead, copper, and mercury. pKa of its aqueous solution are 2.20, 3.72, 4.93, and 9.82. Tetrahydrodomoic acid, m.p. 273° (decomp.), and N-acetyldomoic acid, m.p. 121-123°, were prepared. Neutralization of the latter required 3 equivalents of alkali. Preparation of domoic acid betaine methiodide, m.p. 174°, indicated the presence of three methoxyls. These facts suggest that domoic acid is a tribasic imino acid with two hydrogenatable double bonds.