A unit used in an Australian school to teach English-as-a-Second-Language (ESL) students how to write a discussion is described. The 3-week unit was planned and implemented jointly by an ESL resource teacher, class teacher, and teacher librarian. The class was divided into three heterogeneous groups, two of which were observed for this study and one of which was studied in greatest depth. Texts for student reading, selected by the teachers as models of the discussion genre, and texts written by students during the unit were analyzed. Lessons were also videotaped for analysis of classroom communication. Results indicate how the teacher built and used students' knowledge and guided development of discussion, using a teaching cycle in which her role changed as student learning developed. She had greater input at the beginning of each stage of the cycle, less input towards the middle as students were able to work more independently, and a larger role toward the end as students consolidated knowledge and learning. Techniques used included reading and deconstructing information to inform student writing, introduction of metalanguage to examine the genre of discussion, and use of student knowledge for writing discussion. (MSE)