This study contributes to the limited research on teacher agency in task-based language teaching. Conceptualising agency through the lens of activity theory, the study investigated how two EFL teachers enacted their agency in TBLT implementation at a Chinese university and what factors mediated the enactment of their agency. Data collected by means of classroom observation, interviews and course documents were triangulated to provide a descriptive synthesis of teacher participants' experiences with task-based language teaching. The analysis of the data revealed that these two teachers demonstrated agency through teaching as adaptation and teaching as learning to overcome the contradictions in the instructional activity system. They engaged with both proactive and reactive agency. A number of mediational factors were found to account for their agency. The study highlights the crucial role of teachers' shared beliefs on the value of TBLT and curriculum reform and institutional support in influencing teachers' adoption of task-supported language teaching.