A wide variety of organisms exhibit various circadian rhythms in their behavior and physiology. Circadian rhythms are regulated by internal clocks that are generally entrained primarily by the environmental light:dark (L:D) cycle. There have been few studies of circadian rhythms in fossorial species that inhabit an environment where day-night variations are minimal and where exposure to light occurs infrequently. In this study, circadian patterns of wheel-running activity were examined in naked mole-rats (Heterocephalus glaber). Naked mole-rats are fossorial and eusocial, living in colonies of 60-70 animals with only one breeding female. Most individual mole-rats that ran on wheels (65%) exhibited robust circadian rhythms of locomotor activity, entrained to various L:D cycles, and free-ran in constant darkness (DD) with taus averaging 23.5 h. The remainder of the animals either free-ran or were arrhythmic under the various L:D cycles. Mole-rats generally failed to entrain to non-24-h T-cycles with period lengths ranging from T=23 h to T=25 h. There was considerable inter-individual variation in the circadian patterns of locomotor activity in naked mole-rats as is observed in other subterranean mammals that have been studied. In contrast to the results obtained when mole-rats were individually housed with access to running wheels, circadian rhythms of general locomotor activity were typically not observed for animals monitored while they were housed in a colony setting. However, clear nocturnal rhythms of general locomotor activity were displayed by four males while residing in their home colonies. Two of these males exhibited the physical appearance of a disperser morph - subordinate individuals that are believed to leave their home colonies to achieve reproductive opportunities elsewhere. All four of these males were among the largest males in their respective colonies. These results demonstrate that although naked mole-rats are not frequently exposed to light, the species has retained the capacity to exhibit locomotor patterns of circadian rhythmicity and has the ability to entrain to 24-h L:D cycles. The possible adaptive function of this circadian capacity is discussed.