While ethnically diverse countries are generally believed to be more violent than homogenous ones, previous research has been unable to establish a clear connection between ethnic diversity and violent repression. We argue that political authorities' tendency to violently repress their citizens cannot be explained by the ethnic composition of society per se but by the power distribution between ethnic groups. In a global sample of countries for the period 1977-2010, we find statistical evidence that the level of violent repression increases with the share of the population excluded from state power on the basis of ethnic affiliation. We combine this with a case study of the Republic of Guinea and conclude that political authorities come to see excluded ethnic groups as threats and rely on violent repression to maintain their ethnic dominance.