Growing numbers of students are educated in charter schools, including students with disabilities (SWD). Prior research suggests that charter schools educate a smaller percentage of SWD than traditional public schools, leading to a special education gap between sectors. We used data from a large urban district to examine how choice, mobility, and special education classification influenced the special education gap with descriptive statistics, gap decomposition, and causal analyses relying on a randomized component of school assignment. The driver of the gap was parental choice, with parents of SWD less often choosing charter schools. Enrolling in a charter school had no effect on the probability that a student was newly identified for special education services, but increased the probability that a student was deidentified for special education services. We discuss potential mechanisms underlying these results, with attention to factors that shape parental choice, and future research needed to understand parental choices.