Considering teachers' thinking about a specific mathematical topic allows one to better understand the broader domain of teachers' mathematical thinking and its influence on teaching and learning. This paper explores the meanings and understandings communicated by one middle school mathematics teacher about mathematics and mathematics teaching in general, and fractions in particular, in the context of his attempt to use an innovative set of curriculum materials. Data were collected using interviews, observations, and students' and teacher's written work and plans. The teacher's view of mathematics was of a correct set of rules and concepts and this contributed to his insistence on maintaining a teacher-dominated classroom environment. Also, his own flexible understanding of fractions allowed him to adjust his instruction to accommodate mathematical ideas that were not at the forefront of his own thinking and to emphasize important connections among mathematical representations when provided with curriculum materials to support such an emphasis. Contains 20 references. (MKR)