There is international concern about young people's participation in science. This study investigated the relevant importance of various issues in 1628 Norwegian upper secondary students' choices of postcompulsory subject combinations: natural science and mathematics (henceforth Science) or languages, social science and economics (henceforth HumSoc). Questionnaire items based on the Eccles et al. model of achievement-related choices were grouped into six constructs by factor analysis: expectation of success, interest-enjoyment value, self-realization value, fit to personal beliefs value, utility value for university admission, and relative cost. Interest-enjoyment and fit to personal beliefs were somewhat less important to Science students than to HumSoc students, especially to girls taking Science. Utility value for university admission was much more important to Science than to HumSoc students, and more important to Science girls than to Science boys. Costs in terms of time and effort were much more important to HumSoc than to Science students. The findings indicate that students choose Science both for identity reasons, such as interests, self-realization and fit to personal beliefs, and for strategic utility reasons. Some of the students, especially the girls, appear to have placed more weight on utility than on their interests. Implications for participation in postcompulsory science are discussed. (Contains 5 tables, 1 figure and 4 footnotes.)