Tentacle contact between genetically different individuals of the west coast sea anemone Anthopleura elegantissima initiates in one or both individuals an elaborate behavior pattern that usually results in damage to one or both animals. This aggressive behavior may be described as progressing through five stages: (1) stimulation (or initiation) during which the tentacles of the two anemones repeatedly come into contact and then withdraw from contact; (2) inflation, during which the acrorhagi commence to become turgid; (3) the movement of application, during which the inflated acrorhagi are pushed toward the source of stimulation; (4) application of ectoderm, during which damage is inflicted on the other anemone by the application of acrorhagial ectoderm to the body of the victim; and (5) recovery, during which the anemone returns to its normal posture. The tissue of the victim to which acrorhagial ectoderm has been applied by an aggressor becomes necrotic and sloughs off within a few days. Sections of such tissue that were fixed and sectioned immediately after the application of ectoderm showed penetration of the tissue of the victim by the large atrich nematocysts in the aggressor's acrorhagial ectoderm. The aggressive response of Anthopleura elegantissima is like raptorial feeding and the shell transfer behavior of some anemones in that it is a directed behavior involving asymmetrical movements that are not repeated rhythmically. Aggression resembles the swimming behavior of Stomphia and Actinostola in that the specific sequence of movements, once initiated, continues to completion in the absence of additional or continuing stimulation. The deployment of specialized effector "organs" (the acrorhagi) bearing specialized nematocysts (the large acrorhagial atrichs), the necessity for repeated contact in initiating the response, and the functioning of aggression in intraspecific territoriality are interesting characteristics of this behavior that are unique in the repertoire of anemone behavior. For Anthopleura elegantissima the aggressive response is not directly involved either in defense against predators or capture of prey. Evidence presented here suggests that for this species the aggressive response is involved in intraspecific interaction. Pertinent evidence, presented in this and a previous paper (Francis, 1973) is as follows: (1) Within clonal groups of the aggregating form of Anthopleura elegantissima the individuals are in close contact with their neighbors and the tentacles of the adjacent animals are interlaced. In the field, adjacent clones of these anemones are isolated from each other by anemone-free spaces; and in the laboratory a group of anemones of mixed clonal origins will reaggregate into isolated uniclonal groups (Francis, 1973a). (2) The specificity of the aggressive response is directly related to that shown in the segregation and separation phenomena seen both in the laboratory and in the field. Contact between clonemates does not elicit aggression and is commonly seen both in the laboratory and in the field. Contact between non-clonemates elicits aggression and is not maintained between these anemones either in the laboratory or in the field. (3) The response of the victim of an aggressive episode is such as to affect its separation from the non-clonemate aggressor. The damaged anemone may move away from the aggressor, or sometimes it may merely lean away. In the laboratory, anemones that were severely damaged as a result of repeated aggressive episodes have been observed to relase their hold on the substratum; in the field, such animals would certainly be swept away in the surf. (4) The evidence also shows that aggression does occur at the boundary between two clonal groups. In the field, anemones collected from an interclonal border showed aggression-related damage while their clonemates that were collected from the center of the clonal groups showed no such damage. In the laboratory numerous aggressive episodes were observed over a three week period as two clonal groups that had been brought into artificially close proximity interacted at their common boundary (Francis, 1973a). This suggests that aggression is important in the formation and maintenance of anemone-free zones between adjacent non-clonemates in this species. The direct effects of the aggressive behavior of A. elegantissima would be sufficient to account for clone specific segregation and at least minimal separation between clones; and although other factors, such as the activities of associated animals and the tendency of the young anemones to move apart even in the absence of the aggressive response, may be involved, the aggressive response undoubtedly functions as part of an intraspecific territorial behavior in this species. In Actinea equina also the specificity of the aggressive response correlates with the intraspecific associations apparent in the field; these anemones are solitary and they respond aggressively to contact with any other conspecific. No work has been done on the specificity of the aggressive response in Anthopleura artemisia, however these anemones do respond aggressively to contact with conspecifics, and they have not been found living in contact with genetically different conspecifics. Anthopleura xanthogrammica apparently lacks an aggressive response while having the specialized equipment (acrorhagi bearing atrich nematocysts) used in this behavior. However, the relationship between distribution and behavior is also apparent for A. xanthogrammica since genetically different members of this species are frequently found living in close contact, With the tentacles of adjacent non-clonemates passively interlaced. Taken together, this information suggests that the aggressive response of acrorhagus-bearing anemones is presently effective in intraspecific interactions. It is suggested that the evolutionary origin of the aggressive response among acrorhagus-bearing members...