In some ethno-separatist wars, rebel groups direct a large share of violence against members of their own ethnic community. But why do rebels target the co-ethnics they claim to represent in the war against the government? Our aim in this paper is to provide the components for a conceptual framework that we assess using unique disaggregated casualty data on violence committed by the Liberation Tigers of Tamil Eelam against co-ethnic Tamils in territories claimed for the Tamil Eelam state in the early phase of the Sri Lankan conflict, 1985-88. We propose that there are two distinct processes of intraethnic violence: violence against co-ethnic civilians and violence against co-ethnic rivals. While the former aims at controlling the population to win the war against the government, the latter aims at establishing leadership dominance over the ethnic minority. We examine the role of ethnic homogeneity in shaping the use of violence directed against the two types of co-ethnic targets in the buildup phase of ethno-separatist war. We conclude that ethnic demographic structures matter for how the rebels treat co-ethnics in the early phase of war before they have established territorial control.