Studies of gender disparities in STEM (science, technology, engineering, and mathematics) performance have generally focused on average differences. However, the extremes could also be important because disparities at the top may shape stratification in access to STEM careers, while disparities at the bottom can shape stratification in dropout. This article investigates determinants of gender disparities in math across the performance distribution in Latin American countries, where there is a persistent boys' advantage in STEM performance. Findings reveal disparate national patterns in gender gaps across the performance distribution. Furthermore, while certain national characteristics are linked to gender gaps at the low- and middle-ranges of the performance distribution, female representation in education is the only characteristic associated with a reduced gender gap at the top level.