A quantitative method was used to establish the sexual dimorphism of bone robusticity in 10 domesticated strains of rats and in wild rats. Sexual dimorphism had its highest frequency in the pelvis, followed by the ulna, the skull, and the femur and tibia. Humerus and tail showed the lowest frequency of such sexual dimorphism. This distribution in an unspecialized mammal such as the rat, in which selective pressures are at their minimum because of the factor of domestication, can serve as a baseline for comparison with taxa in which selective forces are more strongly active.