South Africa recorded the largest pro-girl gap in Grade 4 reading and Grade 5 mathematics among participating countries in the 2021 Progress in International Literacy and Reading Study and the 2023 Trends in International Mathematics and Science Study. This paper examines whether these gaps reflect an initial female advantage at school entry, widening gender differences during the early grades, or both. Using parametric and non-parametric methods, we analyse longitudinal administrative data on student marks for more than half a million public school students across six South African provinces from Grades 1 to 4. We find substantial pro-girl gaps across the entire Grade 1 mark distribution in both language and mathematics. In language – and to a lesser extent in mathematics – these gaps widen over the first four grades, particularly in less resourced schools. Girls’ marks move progressively ahead of boys’ marks, with no evidence of convergence between Grades 1 and 4. Divergence is evident across the achievement distribution but is stronger in language than in mathematics. Pro-girl gaps in language and especially mathematics in more resourced schools show less divergence across grades than in less resourced schools. Overall, the findings suggest that South Africa’s large pro-girl achievement gaps in international assessments reflect both an early female advantage at school entry and the cumulative widening of this gap within the schooling system during the foundation and intermediate phases.