This paper reports on a study of the implementation of a new mathematics curriculum and its effect on the teachers implementing it. The study focused on understanding the barriers teachers faced with implementing new curriculum that relies on a different theoretical base than the teachers had previously experienced. Participants in the study were six middle-school teachers and six high-school teachers in a small rural district. Data for the study were gathered from direct observation of the teachers in the classroom and from interviews completed at the beginning of the teachers' second semester using the new curriculum. The study identified the following barriers to the teachers' successful implementation of the new curriculum: (1) concern over the teachers' vision about their roles and beliefs about what their job as a math teacher should be; (2) an assumed student success rate built in by curriculum designers; (3) parents' reactions to their children's test scores; (4) a lack of materials needed to enact the new curriculum; (5) a lack of teacher's technical skills and content knowledge; and (6) factors inherent in the newness of any change. (Contains 26 references.) (WFA)