In this paper, we report the first observational evidence of infanticide in wild hamadryas baboons. The study group inhabits the lowlands of the northern Rift Valley in Ethiopia and has been under observation for over 1200 hours, on and off, since October 1996. Here we report observations from August and September 2002 of the consequences of two takeovers of known females with black infants. After the first takeover, the respective infant disappeared and was presumed dead within eleven days of the takeover. After the second takeover, the infant incurred repeated severe aggression from its mother’s new leader male and eventually died four days after the takeover. We interpret these findings as support for the sexual selection hypothesis for the occurrence of male infanticide. We suggest that hamadryas leader males usually protect infants born into their units but may withhold this protection – or even directly attack and kill infants – after takeovers.