Why aren't there more women in science? This paper begins to shed light on this question by exploiting data from the U.S. Air Force Academy, where students are randomly assigned to professors for a wide variety of mandatory standardized courses.We focus on the role of professor gender. Our results suggest that although professor gender has little impact on male students, it has a powerful effect on female students' performance in math and science classes, and high-performing female students' likelihood of taking future math and science courses, and graduating with a STEM degree. The estimates are largest for students whose SAT math scores are in the top 5% of the national distribution. The gender gap in course grades and STEM majors is eradicated when high-performing female students are assigned to female professors in mandatory introductory math and science coursework.