The sea anemoneAnthopleura elegantissima undergoes a characteristic response when exposed to anthopleurine, a recently isolated alarm pheromone. Aboral longitudinal tentacle muscles and mesenterial retractor muscles undergo a mechanically summed, facilitated series of discrete, rapid, radially-symmetrical contractions, usually followed by a slow, smooth contraction (Figs. 1, 2, 5, 6). The rapid contractions of all retractor muscles are probably simultaneous (Fig. 3). The marginal sphincter contracts at a slower rate (Fig. 2). The alarm response of mesenterial retractor muscles to anthopleurine closely resembles reflex withdrawal in other anemones and is probably mediated by the through-conducting system. Anthopleurine receptors are located primarily on the tentacles. Of ten other species of anemones tested onlyAnthopleura xanthogrammica shows an alarm response to anthopleurine (Table 1).