For the evaluation of sexual dimorphism 739 red fox skulls (including 433 males and 306 females) from the Czech Republic were examined. The individuals younger than six months were excluded from the study of sexual dimorphism and the rest was divided into three age classes (individuals at the age of 6.5–12 months, 12.5–24 months and 24.5 months and older). Skull size differences between males and females were significant in all age classes. Males exceeded females in all dimensions with the exception of postorbital breadth, which was wider in females. Other skull shape differences between males and females were not confirmed. Age class including individuals 12.5–24 months old was the only, in which significant skull shape differences were found. We suppose that competition between males could play the major role in sexual dimorphism formation.