Domoic acid isolated from a seaweed, Chondria armata, showed extremely strong insecticidal activity against cockroaches and houseflies. The minimum effective dose against the American cockroach by subcutaneous injection was 0.8μg/g. This compound also induced significant contraction of the hindgut excised from the American cockroach. The site of action may be the neuromuscular junction, similar to that of α-kainic acid. The structureactivity relationships of domoic and α-kainic acids and their analogs were also investigated. The synthetic analogs were useful tools for delineating the structurally essential requirement for the insecticidal activity.