This study aims to deepen our understanding of teaching, specifically the role of teachers' responsiveness in promoting equity in secondary science teaching. To build a conceptual argument--that teachers' responsiveness expands the opportunity to learn for students from historically marginalized communities--I explore one high school science teacher's classroom instruction using multiple forms of data collected over 2 academic years. The teacher worked with students from Latinx, immigrant, and low-income communities. The data were analyzed focusing on both describing observable teaching behaviors and interpreting their meanings in relation to students' opportunity to learn. The analysis showed that a "responsive" teacher who expanded students' opportunity to learn attended to students' identities, historical relationships, struggles, and ideas. The teacher addressed students' relational challenges in participating in disciplinary practices at the stages of "both" planning and instruction while working against settled hierarchies, cultures, and ideologies reflected in dominant discourses. The theoretical significance and methodological complexity inherent in recognizing teachers' responsiveness for equity are discussed.