This paper introduces a new empirical measure of fiscal fragility, specifically defined by fluctuations in the government’s tax base, to analyze selective sovereign defaults on domestic and foreign debts. Utilizing data from multiple economies, the study tests theoretical predictions linking output and tax base shocks to selective default probabilities. The results confirm that output contractions predominantly drive foreign defaults, whereas fluctuations in the tax base primarily induce domestic defaults. The findings enhance the understanding of sovereign default risks, emphasizing the distinct roles of domestic and foreign debts.