Legal petitions to nullify electoral results comprise a rich source for studying electoral fraud. During a fifty-year period in Costa Rica, parties submitted 120 petitions to Congress, containing more than 1,200 charges of fraud. The petitions reveal that between 1901 and 1938, more than half of such accusations took place in the country's peripheral provinces, where roughly 20 percent of voters lived. They also show that institutional changes helped to shape the nature, frequency, and magnitude of fraud. By the 1940s, the polarization of political competition was accompanied by a geographical redistribution of fraud to the central provinces of the republic, where most of the electorate resided.