The case of Caster Semenya provides a vivid illustration of the ways in which natural genetic variation can generate large differences in athletic performance. But since we normally segregate athletic sports along the lines of this particular variation-gender-her case also highlights problems with the current approach to justice in sporting competition. Female athletes seem to have a valid complaint when they are made to compete against athletes who are, in one sense or another, male. But once we recognise that gender is not a binary quantity, sex segregation in competitive sport must be seen as an inconsistent and unjust policy, no matter what stance we take on the goals of sport or on the regulation of doping.