1. Inter-male relationships were compared between two groups of bonobos at Wamba, Zaire, and a group of chimpanzees at the Mahale Mountains, Tanzania. 2. Although distribution of females is much different between the two species, frequencies of affinitive interactions between males (inter-individual proximity and grooming) were similar between the species. 3. By contrast, marked inter-species differences were found in agonistic aspects of male relationships. Such differences could be related to the different estrus patterns of females. 4. Male relationships of both species may have been reflecting the male-bonded strategy which was taken by a common ancestor. Within this framework, males of each species seem to have developed differentiated social tactics for the access to females showing different patterns of distribution and estrus.