This paper reports evidence on the decomposition of a gender academic achievement gap (GAAP) at the university level by type of college from a unique environment, where the majority of an undergraduate student population shares similar socio-economic and demographic backgrounds and attends the largest and only public university in the country. Results show that between 37% and 45% of the GAAP is due to gender differentials in observable characteristics, and between 55% and 63% is due to the differential in the rates at which productivity-related characteristics are converted into academic achievement. In humanities, the GAAP is a strong reflection of students' performance at the high school level, whereas at scientific colleges it is also explained by the differentials in the returns to their respective abilities, and by the returns to their behavioral attitudes towards university education.