One especially productive avenue of research is the close analysis of classroom talk. In the micro-ethnographic approach used in this study, conversation is not simply a metaphor, but becomes the unit of analysis. For the study, one key element to the functioning of thinking communities is the way they are co-constructed by participants. While all the talk that occurs in any given classroom is potentially relevant to some aspect of learning and achievement, it is the discussion of the subject matter--the content--that can provide crucial insights into the kinds of learning and achievement that are valued and made available in schools. This paper focuses on the subset of data involving exchanges coded by the field team in the classroom as subject matter exchanges; data are drawn from a 5-year longitudinal study of language use in linguistically and culturally diverse subject matter classrooms. The paper explains that, during data collection, research teams of two people were in each classroom, with two video cameras; team members also took observational field notes during classes, and periodically interviewed teachers and students. It notes that the dominant pattern for classroom discourse is teacher-centered, but classroom discourse research has begun to acknowledge student-initiated discourse. The paper finds that, after reviewing the discourse in two high school science classrooms in some detail, the observation is that learning and achievement is accomplished by the construction of thinking communities through quite different discourse practices. It notes some issues relevant to the co-construction of classroom discourse practices across the two classrooms. (Contains 23 references.) (NKA)