This study seeks to develop a better understanding of the underrepresentation of women in science and engineering by analyzing the gender gaps (a) in the interest in pursuing a science degree and (b) on science achievement. We use national-level college admissions data to examine gender differences and to explore the association between these outcomes and the attendance to single-sex or co-educational schools. The Chilean college admissions system provides a unique context to study these gender differences, since applicants who wish to pursue an undergraduate degree in science or engineering are required to take a high-stakes standardized science achievement test as part of the admission battery. This test has three subjects: biology, physics, and chemistry, and applicants must choose to be tested in only one of them. Significant gender differences exist for the examinees in their choice of subject and achievement on (the tests). Gender gaps favoring males are observed in the three forms. Both interest and achievement in science are associated with the sex composition of the school attended.