Close inbreeding is known for a variety of small mammal species(1-4) for which a high probability of mortality during dispersal makes helping and delayed maturation a relatively secure fitness option(5). Prolonged inbreeding, however, is usually associated with lowered fitness(6,7), and it has been shown that most highly inbred small mammals(8) and social insects(9) have inbreeding-avoidance mechanisms that promote some degree of outbreeding. However, previous field and laboratory research on the naked mole-rat (Heterocephalus glaber) suggested that this cooperatively breeding rodent is highly inbred(10-12), with new colonies forming by fission(13). Here we report the discovery of a dispersal phenotype that may occasionally promote outbreeding in naked mole-rats. These dispersers are morphologically, physiologically and behaviourally distinct from other colony members. They are laden with fat, exhibit elevated levels of luteinizing hormone, have a strong urge to disperse, and only solicit matings with non-colony members. These findings suggest that, although rare, a dispersive morph exists within naked mole-rat colonies.