Using data from the Program for International Student Assessment 2006 surveys for 50 countries, this paper explores gender segregation of adolescent science career plans. We ask whether, in different cultures, bridging the male-female gap in science self-concept could reduce gender disparities in students' career preferences. Bringing together the theory of gender essentialism and the biased self-concept thesis, we interpret the cross-national variation in the relationship between self-concept and occupational plans. To this end, we fit a series of random intercept regression models to country-specific and pooled data. In all countries, science-oriented girls prefer employment in biology, agriculture, or health (BAH), whereas boys favor careers in computing, engineering, or mathematics (CEM). Almost everywhere, boys have more confidence in their science ability than girls, even after science performance is taken into account. In advanced industrial countries the male-female gap in science self-concept is larger than the corresponding gap in developing or transforming societies. The male-female segregation of preferences for science careers is also stronger in advanced industrial countries. Nevertheless, nowhere are gender disparities in science self-assessment related to the gender segregation in preferences for BAH and CEM careers. We discuss the significance of these cross-national patterns for science educators and educational policy makers. (Contains 9 tables, 2 figures and 15 footnotes.)