Does the number of political parties influence voter turnout in developing democracies? Some scholars argue that large party systems facilitate matching voter preferences with a specific party, increasing turnout. Others argue multiparty systems produce too many alternatives, decreasing turnout. In developing democracies, there is debate over whether these institutions matter at all. We argue that party systems do matter for turnout in developing countries, but the relationship between turnout and the number of political parties is conditional on the electoral formula. Under proportional representation systems, large numbers of parties increase turnout. Under winner take all systems, large numbers of parties depress turnout. Since electoral rules also influence the number of parties, we use an innovative sub-national research design, taking advantage of local variation in the number of parties that is largely unrelated to the electoral system. Specifically, we test these relationships by analysing turnout data at the municipal level in Brazil and Bolivia, countries with very different electoral rules. Overall, we find evidence that party systems influence turnout, but in different ways depending on the election rules.