Two semifree-ranging groups of ringtailed lemurs (Lemur catta) and two co-ranging groups of redfronted lemurs (Eulemur fulvus rufus) were studied across a two-year period to characterise and contrast the adult agonistic behaviour these primates exhibit within groups. Temporal analyses of behavioural data distinguished agonistic from non-agonistic behaviour and aggressive from submissive behaviour. The ringtailed lemurs employed a diverse repertoire of behavioural elements to communicate agonistic intent. More than 50% of these elements were signals and nearly 50% of signals were submissive. The agonistic repertoire of the redfronted lemurs, by contrast, was relatively unelaborated: less than 40% of agonistic behaviour in this species comprised signals and less than 20% of signals were submissive. These structural differences underlay marked species differences in agonistic interaction and relationship. All pairs of ringtailed lemurs maintained dominance relations resembling those seen in many anthropoid primates: subordinates consistently signalled submissively to dominant partners, often in the absence of aggression. Dominance relations among members of each sex were seasonally unstable and not always transitive (hierarchical) during periods of stability, however. Redfronted lemurs, by contrast, did not maintain dominance relations, failing to respond agonistically to most aggression received (52% of interactions) and responding with aggression on many other occasions (12%). Even applying relaxed criteria, few adult redfronted dyads (14%) showed consistent asymmetries in agonistic relations and several never exhibited any asymmetry. Lacking dominance, E. f rufus relied heavily on alternate behavioural mechanisms to moderate social conflict as frequent and intense as that seen in study groups of ringtailed lemurs. These included a great inclination not to respond agonistically to aggression, a distinctive behavioural proposal to limit or terminate dyadic conflict (Look away), post-conflict reconciliation, and relatively frequent third-party aggression. The existence of such divergent systems of agonistic behaviour in partially sympatric, closely related and generally similar prosimian primates offers important opportunities for comparative study of the ecology, development, and evolution of mammalian social systems. Future research may reveal ecophysiological factors that promote the use of dominance behaviour among like-sexed ringtailed lemurs and show how the relative absence of dominance relates to other major elements of redfronted lemur biology, including 'special relationships' of variable duration between adult males and females.