Problem-based learning (PBL) environment is a student-centered instructional method based on the use of ill-structured problems as a stimulus for collaborative learning. This study tried to gain an understanding of teachers' instructional practices and students' responses to such practices in middle school classrooms with PBL environment through qualitative analyses. A hybrid approach of inductive and deductive thematic analyses was employed and applied to field notes and transcripts of video observations of four PBL classrooms. To do so, a codebook was created based on the descriptions of roles of teachers and students in PBL classrooms in literature, and was then applied to inductive codes that emerged from the data. This study identified a number of specific instructional practices of teachers, as well as responses that students might engage in during PBL instructions. Being able to articulate these roles is an important step in helping new PBL teachers learn to facilitate student-centered classrooms.