This paper (1) describes the first observations of male replacement and infanticide in the blue monkey (Cercopithecus mitis stuhlmanni), (2) examines these observations in light of those hypotheses put forth to explain infanticide, and (3) presents two basic models through which additional hypotheses are developed. Five groups of blue monkeys were observed for 2,724 hr in the Kibale Forest, Uganda. The pattern of infanticide in the blue monkey was strikingly similar to that reported for other species of primates living in one-male bisexual groups. Data concerning infanticide in the blue monkey do not support the hypothesis that infanticide is a maladaptive behavioral pathology. The data indirectly support the hypothesis that infanticide is part of a flexible, adaptive reproductive strategy of new harem-males. According to Model I, two of the hypothese for explaining how infanticide may be adaptive to the perpetrator are not mutually exclusive. Model II suggests that the rate of infanticide is directly related to competition among males for females and indirectly related to tenure length of harem-males. Models I and II underscore the importance of understanding what variables determine tenure length in haremmales. It is cocluded that length of male tenure is most likely a critical determinant of inclusive fitness not only for males but also for females.