This study hypothesized that during high school men and women develop a belief about their competence in science that is based on their participation in certain science courses, their level of performance in those courses, and the effort that they expended. This sense of competence, in turn, affects the science decisions that these students make when they enter colleges, such that students who believe they have ability in science are more likely to choose a science curriculum and pursue a science career than those who do not. Results indicate that although women performed very well compared to men in high school biology and chemistry, their participation and performance in science relative to that of men declined after that point. Women felt that they had worked harder than men did in high school science courses, and they rated their ability lower, even though actual performance was generally better. Results also show that in a path model linking high school and college level variables, this sense of competence in science was a central variable. It is concluded that women's lower sense of competence in science is an important issue in their reduced participation in science courses and careers. (Author/JN)