--- - |2 Abstract Constitutional designers often construct political institutions to provide greater autonomy to ethnic minority groups. One tool available to constitutional designers is ‘ethnic gerrymandering’, where the boundaries of local government units are altered to provide greater representation to minority groups. This paper analyses the effects of changes in the ethnic composition of municipalities, which occur as a result of ethnic gerrymandering, on ethnic party behaviour. I compare ethnic party behaviour in local elections in the Republic of Macedonia from 2000 to 2013. I expand on a theory initially proposed by Sherrill Stroschein linking ethnic demography to ethnic party behaviour. I find that changes in the ethnic composition of municipalities influence whether rival ethnic parties engage in outbidding or whether ethnic communities unite behind a single ethnic party. My findings have important implications for those tasked with designing political institutions in ethnically divided societies. - 'Nations and Nationalism, EarlyView. '