The purpose of this qualitative case study was to investigate the development of social norms in a fifth-grade reform-based mathematics classroom through a close examination of one teacher's actions during the first three weeks of school. This study offers insights into the following research questions: (1) How does an elementary teacher in a reform-based mathematics classroom negotiate the development of social norms through the context of mathematics at the beginning of the school year? (2) What is the role of social norms in the development of sociomathematical norms? (3) In what ways do teacher moves support or hinder the development of sociomathematical norms? The main data sources were videotaped classroom observations and teacher interviews. Data analysis involved a two-cycle coding process utilizing several coding strategies. The work of Kazemi and Stipek (2001) was used in conceptualizing the categories of social norms found in the data. The findings describe the context of the classroom; the teacher's beliefs, values, and attitudes towards mathematics; and how the teacher set the stage for learning mathematics. There were four specific social norms found in the data that had the potential to become building blocks for sociomathematical norms: (1) students collaborate to solve problems, (2) students view mistakes as a natural part of the learning process, (3) students share strategies and explain their thinking, and (4) students solve problems using a variety of strategies and representations. The teacher negotiated the development of social norms through his choice of high-cognitive demand tasks and collaborative grouping structures as well as pedagogical moves that held students accountable and probed their mathematical thinking. This happened in the context of a supportive classroom environment where students were willing to take risks and talk about their mathematical thinking. The teacher's role as a mathematical authority in the classroom, his use of questioning that sometimes diminished students' thinking, and his language that communicated mixed messages regarding the nature of mathematics may have hindered the development of productive sociomathematical norms. This study has implications for preservice and inservice teachers as well as administrators and professional development leaders in the field of mathematics education. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]