The purposes of the present study were to develop and evaluate a measure of self-efficacy expectations with regard to the performance of mathematics-related behaviors and to investigate the relationship of mathematics self-efficacy expectations to the selection of science-based college majors. Based on results obtained from a pilot sample of 115 college students, 52 math-related tasks were selected from an initial 75-item pool. Subjects, 153 female and 109 male undergraduates, were asked to indicate their degree of confidence in their ability to successfully perform the tasks or problems or to complete the college course with a grade of "B" or better. As predicted, the mathematics-related self-efficacy expectations of college males were significantly stronger than were those of college females, particularly with regard to mathematics-related college courses. Mathematics self-efficacy expectations, but not any mathematics performance index, contributed significantly to the prediction of the degree to which students selected science-based college majors, thus supporting the postulated role of cognitive mediational factors in educational and career choice behavior. The utility of the concept and measure of mathematics self-efficacy expectations for the understanding and treatment of mathematics anxiety and mathematics-avoidant behaviors is discussed. (Author/MP)