Using the technique of DNA fingerprinting, we investigated the genetic structure within and among four wild-caught colonies (n = 50 individuals) of a eusocial mammal, the naked mole-rat (Heterocephalus glaber; Rodentia: Bathyergidae). We found that DNA fingerprints of colony-mates were strikingly similar and that between colonies they were much more alike than fingerprints of non-kin in other free-living vertebrates. Extreme genetic similarity within colonies is due to close genetic relationship (mean relatedness estimate +/- SE, r = 0.81 +/- 0.10), which apparently results from consanguineous mating. The inbreeding coefficient (F = 0.45 +/- 0.18) is the highest yet recorded among wild mammals. The genetic structure of naked mole-rat colonies lends support to kin selection and ecological constraints models for the evolution of cooperative breeding and eusociality.