The involvement of dopamine within the nucleus accumbens in the formation and maintenance of pair bonds was assessed in a series of experiments using the monogamous prairie vole. We show that dopamine transmission that promotes pair bond formation occurs within the rostral shell of the nucleus accumbens, but not in its core or caudal shell. Within this specific brain region, D1- and D2-like receptor activation produced opposite effects: D1-like activation prevented pair bond formation, whereas D2-like activation facilitated it. After extended cohabitation with a female, male voles showed behavior indicative of pair bond maintenance-namely, selective aggression towards unfamiliar females. These voles also showed a significant upregulation in nucleus accumbens D1-like receptors, and blockade of these receptors abolished selective aggression. Thus, neuroplastic reorganization of the nucleus accumbens dopamine system is responsible for the enduring nature of monogamous pair bonding. Finally, we show that this system may also contribute to species-specific social organization.