Three novel amino acids with insecticidal activity, isodomoic acids A, B and C, were isolated from water extracts of the red alga Chondria armata. Domoic acid (I) from C. armata is strongly insecticidal against the American cockroach (Periplaneta americana) when injected subcutaneously and induces significant contraction of its excised hindgut, prompting re-examination of the alga's constituents. The water extract was fractionated by means of an assay based on insecticidal activity against the American cockroach, and three novel amino acids closely related structurally to domoic acid (I) were isolated from the insecticidal fraction. Their structures, named isodomoic acid A (III), B (IV) and C (V), were elucidated by spectroscopic analysis including 1H-1H shift correlated spectroscopy (COSY), NOE difference experiment, and 13C-NMR spectrum, establishing their stereostructures. Isodomoic acids A, B and C were significantly insecticidal when injected subcutaneously into American cockroaches with minimum effective doses of 3.2 × 10⁻⁸, 3.2 × 10⁻⁸, and 6.4 × 10⁻⁸ mol, respectively.