Effects of schooling on a geometric misconception were examined by comparing the performance of Israeli students attending ultraorthodox schools with that of peers attending mainstream schools. These groups were of special interest because both value education highly and send essentially all children to school, but I group receives extensive instruction in mathematics and science and the other receives almost none. Despite the ultraorthodox 12- to 14-year-olds' having received no instruction in geometry, they more often solved the geometric misconception problems than did mainstream peers who had received extensive instruction in the subject. Mainstream 16- to 18-year-olds did somewhat better on the misconception task than did orthodox age peers, but even there, the advantage of the mainstream students was limited to those exposed to the most advanced mathematics curriculum. How mainstream and orthodox schooling may have contributed to these findings is discussed.