Evidence is presented from an ethnographic study of an eighth grade pre-algebra teacher's classroom in support of the idea that teachers' beliefs about the ontology and epistemology of math profoundly influence how they teach and thus what students learn. Specifically, beliefs based on a traditional view of mathematics as procedurally-based are explored. Classroom observations and teacher interviews provide evidence that expert, rule-based instruction may be beneficial in providing strong foundations for teacher and student efficacy, a clear belief system about the nature of mathematics, unambiguous guidelines for instruction, including areas such as scaffolding and assessment, and specific methods for classroom management. On the other hand, procedural instruction in this classroom was related to student misconceptions, unremarkable mathematics achievement, excessive student dependence on the instructor, and extrinsic student motivation. The implications of this analysis for implementing mathematics reforms based on the differing metaphors of learning are explored. Contains 42 references. (Author/WRM)