The use of active learning in the undergraduate biology classroom improves student learning and classroom equity, but its use can lead to student anxiety. Instructors can reduce student anxiety through practices that convey supportiveness and valuing of students. We collected students' ratings of their classroom anxiety and perceptions of their instructors' supportiveness, as well as open-response reasons for their ratings, in six large introductory biology classes. These data confirmed a negative relationship between student anxiety and student perceptions of their instructors' support. We used qualitative analysis to identify themes of instructor support and how these themes varied between instructors rated as providing higher or lower support by their students. Two instructors with higher-support ratings and two with lower-support ratings were selected for analyses. Inductive qualitative coding identified five themes of instructor support: "relational" (perception of caring/approachability), "instrumental" (offering resources), "pedagogical" (quality of teaching), "personality," and "uncertain" (not sure of support). Higher-support instructors had more positive relational themes and fewer negative pedagogical themes compared with lower-support instructors. These results can be used to enhance supportive classroom practices, which may be one mechanism to reduce student anxiety.