In three adjacent troops of olive baboons (Papio anubis), all males emigrated from their natal troop. There is evidence that the costs of inbreeding depression may exceed those of transferring to another troop. Migrations which were unlikely to be due to the avoidance of inbreeding were made by males with above average reproductive activity into troops with more oestrous females than the prior troop. During inter-troop encounters, immigrant ('transferred') males acted in ways that reduced the contact of females of their own troop with outsiders and lowered the probability of newcomers joining the troop. In contrast, 'natal' males showed interest in members of the other troop. Male dominance was strongly related to age, and in transferred males reproductive activity was related to dominance. In natal males dominance increased with age but reproductive activity decreased. Instead of engaging in aggressive competition for access to oestrous females, natalmales mated surreptitiously. Females showed a preference for transferred males over natal males, and for males who could not have been their father over males who could have been.