A longitudinal study was conducted to test the mediational role of efficacy expectations in relation to sex differences in the choice of a math/science college major. Data on 101 students were gathered prior to their entering college and then again after they had declared a major 3 years later. Path analytic results support the importance of both math self-efficacy beliefs and vocational interest in mathematics in predicting entry into math/science majors and mediating sex differences in these decisions. Also, students who described themselves as more extroverted were less likely to take additional math classes in high school. Students with stronger artistic vocational interests chose majors less related to math and science. School personnel are strongly encouraged to develop programs that challenge the crystallization of efficacy beliefs and vocational interest patterns before students enter college.