This article uses data from the U.S. High School Longitudinal Study of 2009 to investigate the relationship between high school students' beliefs about female abilities in math and science and their teacher gender, beliefs, and classroom behaviors. Estimates are obtained by comparing the same ninth graders between math and science classes, thus controlling for student fixed effects. Students were less likely to believe that men were better than women in math or science when assigned to female teachers or to teachers who valued and listened to ideas from their students. The empirical analysis also provides evidence suggesting that these gender beliefs were related to the decisions by female students to take advanced math and science classes in high school.