The sea anemone Anthopleura elegantissima responds with characteristic contraction to a pheromone released by wounded conspecifics. The alarm pheromone was isolated by ion-exchange chromatography and identified by chemical and spectroscopic methods as the quaternary ammonium ion (3-carboxy-2,3-dihydroxy- N , N , N -trimethyl)-1-propanaminium. Median effective concentration of the crystalline pheromone is 3.5 × 10 -10 mole per liter of seawater.