Inquiry-based science instruction (IBSI) has the potential to contribute to social justice through widening participation and success in science. However, teachers struggle to implement IBSI because of contextual factors. The purpose of this paper is to explore the use of agency as a framework for understanding teachers' decision-making, by asking the question: what was the agency of six science teachers in their social-justice aspirations and use of IBSI? An agency framework recognizes that teachers' decisions are informed by their internal conversations at the intersection of personal aspects (their history, repertoire, and aspirations) with the cultural and structural constraints and resources of their contexts. However, research on IBSI has focussed on teachers' personal aspects or their perceptions of contextual factors. The sample had learnt IBSI through service learning in the context of a science fair. From teacher interviews, it emerged that these teachers had strong social-justice aspirations to serve disadvantaged students. The teachers linked their social-justice aspirations to their choice of school rather than their use of IBSI. The teachers at better-resourced schools initiated participation in science fairs at their schools, whilst the rest judged that science fairs were not appropriate for their students. The results suggest that, rather than prescribing particular pedagogies, teacher education programmes should aim to increase teachers' pedagogical repertoires, in order to enrich their agency in whatever contexts they teach.