Background: Choice behaviour has far-reaching consequences on students' educational careers. Previous models on course selection for example, the model of achievement-related choices (Wigfield & Eccles, 2000) and of self-efficacy theory (Bandura, 1997), stress the importance of ability perceptions (self-concept of ability) as major determinants of choice. Aim: This article suggests a model of course selection, which assumes that comparisons within an internal frame of reference (as proposed by Marsh, 1986, for the association between achievement and self-concepts) can also be applied to the association between self-concepts and course selection. It is therefore hypothesized that course selection is not only positively influenced by the self-concept with respect to the corresponding subject, but also negatively with respect to alternative subjects. Moreover, it should be tested whether the effects of previous achievement on course selection are completely mediated by the self-concepts. Sample: These assumptions were empirically tested using a sample of 296 students from secondary school classes, who were specializing in, for example, chemistry or biology in the following school term. Method: Self-concepts and course selection were assessed via questionnaire. The postulated models were tested using a structural equation modelling approach for ordinal variables. Results: The core assumption, that course selection is determined by dimensional comparisons, was supported by significant negative paths from self-concepts on the selection of non-corresponding subjects. Furthermore, the effects of previous achievement on selection were completely mediated by self-concepts. Conclusion. Previous models of choice behaviour should be extended, by considering not only the selected alternatives, but also the unselected ones. The finding that the influence of achievement on choice is completely mediated by self-concepts, demonstrates that subjective interpretations of previous achievement determine subsequent behaviour.