Close inbreeding is known for a variety of small mammalspecies for which a high probability of mortality duringdispersal makes helping and delayed maturation a relativelysecure fitness option. Prolonged inbreeding, however, isusually associated with lowered fitness, and it has beenshown that most highly inbred small mammals and socialinsects have inbreeding-avoidance mechanisms that promotesome degree of outbreeding. However, previous field andlaboratory research on the naked mole-rat (Heterocephalusglaber) suggested that this cooperatively breeding rodent ishighly inbred, with new colonies forming by fission. Here we report the discovery of a dispersal phenotype that mayoccasionally promote outbreeding in naked mole-rats. Thesedispersers are morphologically, physiologically andbehaviourally distinct from other colony members. They areladen with fat, exhibit elevated levels of luteinizing hormone,have a strong urge to disperse, and only solicit matings withnon-colony members. These findings suggest that, although rare,a dispersive morph exists within naked mole-rat colonies.