Whether the opposition in a democratic country is unified or fragmented is an important characteristic that may influence the accountability of government, as a recent study demonstrates that the degree of opposition fragmentation has consequences for the electoral performance of ruling parties. This study explores the determinants of opposition fragmentation using data from 18 advanced democracies. I argue and provide evidence that deliberative parliamentary rules that allow opposition parties to have greater influence in the policymaking process lead to higher levels of opposition fragmentation. Opposition parties deprived of political influence tend to reduce their levels of fragmentation in order to become more competitive. This finding suggests that there may be a trade-off between deliberativeness and competitiveness in democratic politics.