Among “savanna” baboons, males are the dispersing sex, and females are philopatric. Despite clear evidence for migration of adult males at Erer-Gota, Ethiopia (Abegglen, 1984), it is generally believed that a different pattern-dispersal only by female transfer-is found in hamadryas baboons,Papio hamadryas hamadryas (Pusey and Packer, 1987; Pusey, 1988; Stammbach, 1987). Since the late 1960's, there have been isolated observations of hamadryas males migrating into anubis groups in the Awash National Park, Ethiopia (Nagel, 1973; Kawai and Sugawara, 1976; Sugawara, 1982). Since 1983, we have observed 11 individually identified adult hamadryas immigrants in four anubis groups above the Awash Falls and have trapped and tagged 9 of them. One subadult male was also captured and marked. Repeated visits to the study site allow us to document long-term residence of these “cross-migrant” males in their host groups. The longest-resident male has been in the same group for 5 years or more; a conservative estimate of the average length of residence is 3 years. We estimate that 25 hamadryas males have moved into this ozne over the last 15 years. Although larger than the hamadryas males captured in 1973, all but one of our cross-migrants appear phenotypically hamadryas. By comparing, the ages of our cross-migrants with Abegglen's account of the typical hamadryas male life-history, we have found that the adult hamadryas males seem to immigrate at ages consistent with having left their