Sustainability issues have led to increased demands for a STEM-literate society and workforce. Potential contributors need to be competent, have an understanding of earth and physical sciences, and be willing to pursue such fields. High school girls, however, remain underrepresented in physical science course enrollments (College Board, 2014). This qualitative case study examined how gender informs girls' efficacy-activated processes related to their perceptions of potential science course pursuits. From a feminist, social cognitive theoretical framework, it sought to understand how various factors, including gender, interplay in girls' determinations of their perceived STEM self-efficacy and potential science course trajectories. Findings illuminated how gender role socialization, particularly career orientations and competitive norms, threatens girls' participation in physical sciences. Implications for policy, research, and practice are discussed.