This article reports on a qualitative multiple case study that explored the academic discourse socialization experiences of L2 learners in a Canadian university. Grounded in the notion of "community of practice" (Lave & Wenger, 1991, p. 89), the study examined how L2 learners negotiated their participation and membership in their new L2 classroom communities, particularly in open-ended class discussions. The participants included 6 female graduate students from Japan and 10 of their course instructors. Student self-reports, interviews, and classroom observations were collected over an entire academic year to provide an in-depth, longitudinal analysis of the students' perspectives about their class participation across the curriculum. Three case studies illustrate that students faced a major challenge in negotiating competence, identities, and power relations, which was necessary for them to participate and be recognized as legitimate and competent members of their classroom communities. The students also attempted to shape their own learning and participation by exercising their personal agency and actively negotiating their positionalities, which were locally constructed in a given classroom. Implications for classroom practices and future research are also discussed. (Contains 3 tables and 7 footnotes.)