An ethnographic approach is taken to describe the development of student critical thinking in the Pittsburgh Discussion Project, focusing primarily on discussion experiences in three classes (one college-bound 9th-grade class and two mainstream 11th-grade classes) over the school year. Weekly discussions were audiotaped, and interviews were conducted with teachers and students. The study was framed by three questions: (1) How does classroom social context affect student thinking and collaboration? (2) How do students collaborate to develop interpretations of texts? and (3) To what extent are critical thinking and collaboration evident in discussion? This paper focuses on how the classroom context for discussion varied from teacher to teacher and how student thinking was fostered or discouraged in each class. In the two classes where students were led to see the text as open to alternative interpretations, student questioning became the major spur for discussion and critical thinking. In the other class, where the teacher considered her interpretations of the text to be authoritative, the students soon learned that their interpretive responses and critical thinking had no place. The findings of the study provide evidence for a view of discussion as a "sociocognitive activity" (J. A. Langer, 1987), meaning that out of the context for using language in social interaction, students learn particular ways of thinking that affect the meanings they produce. (JD)