This cross-case ethnographic study examines genres of discussion in two public high school English courses to explore the interplay between dialogism, structure, and critical and collaborative thinking practices. Bakhtin's concepts of dialogism and speech genres as well as Vygotsky's concepts of thinking and language and the zone of proximal development provide the theoretical premise of this research. Data sources included field notes, audio recordings and transcriptions, artifacts of the teacher's handouts and students' written work, informal conversations, and an interview with the teacher. I used discourse analysis and grounded theory to analyze the data, looking at both lively and problematic episodes of discourse. An honors 12th grade class is juxtaposed with a lower-level ninth grade class as a teacher's choices about meta-talk, degrees of structure, and genres of dialogic discussion are described. In the honors class, the teacher uses three genres of discussion: warm-up, book gossip, and deeper-level thinking. These genres create openings for dialogic discussion and invite students to participate in collaborative critical thinking practices. In the lower-level class, the teacher uses prereading instead of warm-up, and she uses a greater degree of structure and authority to invite students to use deeper-level thinking practices. This study finds that the use of structure may support or obstruct deeper-level thinking. Meta-talk differs greatly between the two courses, highlighting the negative impact of high-stakes testing as part of states' implementation of the Common Core State Standards and illustrating the impact of within-school ability sorting on classroom cultures. The study finds that dialogic talk can support critical and collaborative thinking practices with both levels of students; however, a fluid and responsive approach to structure is necessary to support students while providing them with flexibility to create their own paths of thought. Using genres of talk as part of a dialogic approach to teaching can communicate teachers' intentions for dialogism and critical thinking and teach students to learn through collaboratively building meaning. These findings suggest strategies and aspects of the teacher's stance that can support students as they learn to think critically and collaboratively.