Self-concept and self-efficacy are two of the most important motivational predictors of educational outcomes. As most research has studied these constructs separately, little is known about their differential relations to peer ability, opportunities-to-learn in classrooms, and educational outcomes. We investigated these relations by applying (multilevel) structural equation modeling to the German PISA 2006 data set. We found a correlation of ρ = .57 between self-concept and self-efficacy in science, advocating distinguishable constructs. Furthermore, science self-concept was better predicted by the average peer achievement (Big-Fish-Little-Pond Effect), whereas science self-efficacywas more strongly affected by inquirybased learning opportunities. There were also differences in the predictive potential for educational outcomes: Self-concept was a better predictor of future-oriented motivation to aspire a career in the sciences, whereas self-efficacy was a better predictor of current ability. The study at hand provides strong evidence for the related but distinct nature of the two constructs and extends existing research on students’ competence beliefs toward social comparisons and opportunities-to-learn. Further implications for the relevance of inquiry-based classroom activities and for the assessment of competence beliefs are discussed.