Periodically, members of captive Lemur social groups target others for intense aggression. Over periods of several days, weeks, and sometimes months, one to three lemurs persistently follow and attack one or two particular group mates until the targets no longer associate with their group. Episodic targeting aggression is nonrandom with regard to time of year, group size and sex ratio, and the kinship, age, and gender of targets. The vast majority of episodes observed over the past 18 years at the Duke University Primate Center (DUPC) has occurred between like-sex adolescents and adults in conjunction with estrous cycling or infant births and most often after groups have reached apparent critical sizes. When unrelated adults have shared group membership, members of one family have almost invariably first targeted members of the other. In several groups, entire matrilines have gradually been evicted by members of another across periods of several years. When non-relatives have been absent, lemurs have evicted relatives that had previously formed separate subgroups. Episodic targeting aggression has been documented at the DUPC in over a dozen different social groups, comprising three different species. The phenomenon occurs repeatedly in groups held in a variety of large outdoor runs as well as in outdoor enclosures providing naturalistic space and physical structure. More-over, an appreciable number of recent observations in Madagascar suggest that the patterns we have documented well represent the phenomenon as it occurs in the wild. Targeting aggression based on group size, sex ratio, kinship and gender has been reported for no other primate taxon. We suggest that episodic targeting aggression reflects heretofore undescribed tactics in reproductive competition that may characterize many lemurid and indriid taxa. As such, the phenomenon has broad implications for the structure of lemur social groups and populations. Provisional models of the social dynamics and histories of Lemur social groups are presented for evaluation during upcoming field work.