Social cognitive career theory posits that proximal contextual support variables exert both direct and indirect effects on individuals’ career choice intentions. The purpose of this study was to test this proposition by examining the hierarchical structure of efficacy beliefs and their relations to students’ intentions to pursue careers in science. Data were collected from a sample of undergraduate students (N = 1,693) recruited from biology, chemistry, and physics courses. Results of multilevel modeling analyses indicated that aggregated peer science self-efficacy (PSSE) perceptions in the classroom were positively predictive of science career intentions (SCIs) while holding constant the Level 1 influence of science self-efficacy. Moreover, science interest was shown to mediate the relationship between PSSE and SCIs. Implications for career development research and practice are discussed.