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The size and composition of primate groups affect dispersal and reproductive opportunities, and thus have implications for the conservation and management of endangered species. Group fissions that increase dispersal options can reduce close inbreeding, particularly in small, isolated populations that are candidates for translocation and other forms of management. While the impact of group size on permanent group fissions has been well documented in matrilocal societies, less is known about the process of permanent fission in patrilocal primates. Here, we use social network analyses of four years of scan-sample data (2011–2014) to retrospectively investigate whether membership in a newly fissioned group of northern muriquis (Brachyteles hypoxanthus) at the Reserva Particular do Patrimônio Natural – Feliciano Miguel Abdala, Caratinga, Minas Gerais, Brazil, could be predicted from prior social relationships. Modularity metrics indicated the start of the Matão group fission during the 2012–2013 mating season, with a distinct subgroup of individuals that formed the new Nancys group being evident. Dyadic associations were significantly stronger and eigenvector centrality values were significantly higher among the individuals that formed the Nancys group than among those that remained in the Matão group. One mother and her four adult sons were among the founders of the Nancys group, but other mother-adult son families did not fission together. Our analyses add to the literature on permanent fissions in patrilocal societies. They also suggest that the modularity metric of social network analyses can be useful in identifying the best timing for implementing management programs that take social preferences into account. Such considerations can contribute to informed decisions about translocations, reintroductions, and the rehabilitation of groups or subgroups in the conservation management of endangered primates such as muriquis. [ABSTRACT FROM AUTHOR]