Teacher agency is considered key in shaping teachers' professional identities and decision-making capabilities. We suggest that the concept of agency also constitutes a useful tool for evaluating the successful implementation of new teaching approaches. In this paper we discuss findings from a teacher professional development programme aimed at enabling science capital building approaches in the classroom. By applying the lens of agency we identified developments in teachers' sense of purpose, mastery, reflexivity and autonomy. We also identified factors which appeared to either promote or constrain the acquisition of agency. The science capital pedagogy is underpinned by a social justice agenda. In supporting teachers to "tweak" their practice in ways that provide more opportunities for more students to identify with science, we suggest that the agency fostered by the intervention may best be defined as critical teacher agency.