Many studies have investigated whether students in charter schools differ systematically from those in traditional public schools with respect to prior achievement, special education, or English Language Learner status. None, however, has examined gender differences in charter school enrollment. Using data for all U.S. public schools over 11 years, we find charters enroll a higher fraction of girls, a gap that has grown steadily over time and is larger in secondary grades and KIPP schools. We then analyze longitudinal student-level data from North Carolina to examine whether differential rates of attrition explain this gap. We find boys are more likely than girls to exit charters once enrolled, and gender differences in attrition are larger than in traditional schools. However, the difference is not large enough to explain the full enrollment gap between charter and traditional schools in North Carolina, suggesting gaps exist from initial matriculation.