Objectives : Although conflicts between groups over valuable resources are common in the animal kingdom, an individual's strategy toward out-group individuals may differ according to the benefits and costs received from inter-group interactions. Groups of bonobos encounter each other frequently and may mingle and range together from a few hours to a few days. During these inter-group associations, individuals across groups exhibit both aggressive and affiliative interactions. This study aimed to examine the strategies that bonobos employ with other groups, by comparing the patterns of within-and inter-group aggression. Materials and methods : We observed the aggressive interactions within a group of wild bonobos and between the group and three neighboring groups in Wamba, Luo Scientific Reserve, DR Congo. Results : Bonobos increased the level of cooperation to attack out-group individuals more than they do to attack within-group individuals. Additionally, they reduced the aggression between within-group members during inter-group associations, compared to that when not associated with other groups. Males selectively and cooperatively attacked out-group males. Inter-group aggression among females was rare. Furthermore, females sometimes formed coalitions with out-group individuals to attack a common target. Discussion : Our results support the hypothesis that inter-group competition occurs in bonobos, with males across groups competing over mates. Females across groups were tolerant and even cooperative with each other. Regardless of the ideal male strategy, female tolerant and cooperative relationships across groups and female within-group superiority over males could preserve tolerant inter-group relationships in bonobos.