Purpose: Teacher agency plays a crucial role in teachers' professionalism and makes them capable of acting powerfully in their teaching context. The purpose of this study was to explore the role of case-based instruction (CBI) on the agency development of EFL (English as a foreign language) student-teachers. Design/methodology/approach: The study followed a descriptive narrative design and involved ten student-teachers in five CBI sessions. The data collected through different data sources, such as narratives, interviews, recorded sessions and focused groups, were analyzed using a deductive-inductive thematic analysis. Findings: The results showed that CBI has a constructive role in developing the agency of EFL student-teachers. Active participation in CBI sessions helps teachers consider their future desires. The exploration of cases in CBI sessions helps them recall past experiences that led to the development of their agency. Brainstorming ideas in CBI sessions helps teachers consider their beliefs in decision-making and analyze their past experiences. In conclusion, CBI can help develop teacher professionalism and acknowledgment of the importance of teacher agency. Originality/value: The research has addressed student-teachers' agency development through an ecological perspective and CBI.