Either during or shortly after sexual maturation, male rhesus monkeys leave the social groups in which they were born and join other non-natal groups (refs 1, 2 and unpublished data). Here we report that males frequently transfer into the same social groups as their older maternally related brothers. Such brothers spent more time close to each other than to other males, formed alliances with brothers more frequently than with non-brothers and disrupted each other's interactions with oestrous females less frequently than expected by chance. They also spent more time in non-natal groups than did males who did not have a brother in the group. Length of time in a group is positively associated with male dominance rank1 and probably with reproductive success. The data are consistent with proposals that brothers who behave nepotistically and noncompetitively have high inclusive fitness.