This study compares and contrasts the classroom social norms of two Korean and two U.S. second grade teachers who aspired to implement reform. Despite similar social participation structures, the two classes within each country were remarkably different in terms of sociomathematical norms. In one class in each country, students experienced mathematics on the basis of a rather fixed procedure that the teacher consistently emphasized. The students' mathematical ways of thinking and valuing were limited to finding out the pre-determined rules. In contrast, the students in the other classes learned mathematics on the basis of their own sense-making processes. A specific solution method or idea was scarcely emphasized over students' various ways of approaching a given mathematics problem. The students were continually engaged in significant mathematical processes by which they could develop an appreciation of characteristically mathematical ways of thinking, communicating, arguing, proving, and valuing. The similarities and differences between the two teaching practices within each country clearly indicate that students' learning opportunities do not arise from the general social norms of a classroom but from its sociomathematical norms. This study also supports the growing realization of the reform community that reforming mathematics teaching is neither a matter of changing the social structure of instruction nor of adding a few new techniques to an existing repertoire. (Contains 57 references.) (ASK)