Sense of belonging to math--one's feelings of membership and acceptance in the math domain--was established as a new and an important factor in the representation gap between males and females in math. First, a new scale of sense of belonging to math was created and validated, and was found to predict unique variance in college students' intent to pursue math in the future (Studies 1–2). Second, in a longitudinal study of calculus students (Study 3), students' perceptions of 2 factors in their math environment--the message that math ability is a fixed trait and the stereotype that women have less of this ability than men--worked together to erode women's, but not men's, sense of belonging in math. Their lowered sense of belonging, in turn, mediated women's desire to pursue math in the future and their math grades. Interestingly, the message that math ability could be acquired protected women from negative stereotypes, allowing them to maintain a high sense of belonging in math and the intention to pursue math in the future.