The anemones in a single continuous aggregation of the sea anemone Anthopleura elegantissima are observed to resemble each other in two obvious ways: (1) all have the same color pattern, and (2) all individuals having developed gonads are of the same sex. Furthermore, while sustained contact between individuals within an aggregation is very common, contact between individuals from different aggregations has been found to initiate an intraspecific aggressive response resulting in the separation of these individuals (Francis, 1973). These animals are known to reproduce asexually by longitudinal fission, and it is therefore concluded that the aggregations must each be comprised of anemones from a single clone. It seems to me improbable that anything other than genetic identity could account for aggregations of up to thousands of individuals of the same sex, having the same color patterns and showing mutual tolerance for contact with each other and for no other members of the species. Even among anemones within a single clone, the presence of gonads containing gametes is shown to be related to size. Anemones smaller than 0.2 g dry wt consistently lack developed gonads, while some proportion of those over 0.2 g have gonads with gametes. In the field, adjacent clonal groups are observed to remain separated from each other. In the laboratory a mixed group of anemones will reaggregate into isolated uniclonal groups, implying that clone specific segregation is actively accomplished by the anemones. Living in aggregations has a number of potential advantages for the anemones such as reducing their effective surface area and thereby reducing water loss and the battering effects of wave action, excluding interspecific competition for space by promoting effective blanketing of an area, and allowing for cooperation in the capture and holding of larger prey. The function for the anemones of clone specific segregation and separation is not yet clear (see Francis, 1973), however the phenomenon is not without parallel since some other coelenterates as well as some bryozoans have also been observed to respond differently to contact with a genetically identical conspecific than to contact with other conspecifics.