© 2016 Wiley Periodicals, Inc.In many primate species that form one-male breeding units (OMUs), the threat of a takeover by a bachelor male represents a major challenge to group stability and individual reproductive success. In the case of snub-nosed monkeys, which live in large multilevel or modular societies (MLS) comprising several OMUs that travel, feed and rest together and as well as one or more all male units (AMUs), the process by which rival males challenge resident OMU males for access to females is poorly understood. From September 2012 to October 2013, we recorded 48 cases in which rival males visited an OMU in a MLS of Yunnan snub-nosed monkeys (Rhinopithecus bieti) inhabiting the Baimaxueshan National Nature Reserve, Yunnan Province, China. In 40 cases, rival males engaged in mild agonistic interactions (approaching, staring, teeth-baring and chasing) but failed to take over the group; we counted these visits as failed takeovers, recognizing that they may nevertheless allow rival males to assess the competitive ability of residents. During eight successful takeovers, however, there was severe physical aggression between challenging and resident males, with serious injuries to participants. We found that neither the number of adult and subadult females in an OMU, the number of non-pregnant, non-lactating adult females in an OMU, nor the rank of a resident male relative to other resident males in the MLS predicted which OMU a challenging male targeted for takeover. However, a resident male's rank significantly predicted whether takeover attempts were successful. Specifically, challenging males were more successful in displacing a lower-ranking resident male than a higher-ranking male. Given that a Yunnan snub-nosed monkey MLS may contain as many as 40 resident and 36 bachelor males, continued research is required to determine the set of factors that enable resident males to maintain high social rank and successfully defend their harems. Am. J. Primatol. 78:974–982, 2016. © 2016 Wiley Periodicals, Inc.