bstraction at which these rival ethnic groups were sensitive to demographic trends, and therefore the level at which that rivalry played out, was the local municipality, the opstina where competition over jobs and political power was manifest. It is at this level that basic tenets of ethnic competition theory are considered to have been at work. An index of ethnic competition is introduced to measure competition in terms of the relative balance of ethnic populations. This index is complemented by analysis of the trend of relative ethnic population numbers over the two decades prior to the war. Data from the Yugoslavian census show how the demographic position of the Bosnian Serbs declined dramatically in over 90% of the opstinas throughout the country. These population trends are translated into an index of demographic disadvantage.