This study is an attempt to document the connection between the way children construct mathematics in the second grade and the way student teachers construct their knowledge base. The study examines how student teachers integrate their formal and informal knowledge of mathematics and mathematics pedagogy with the reality of the second grade classroom. Preservice teachers (N=4) taught in a classroom where mathematics instruction was conventional and in a classroom where mathematics was taught from a constructivist perspective. The research study includes a qualitative documentation and analysis of the instructional approach, the children's mathematical understandings, and the student teachers' knowledge. The results of this case study address the issue of how student teachers in two different mathematical environments construct their knowledge about key questions related to the nature of mathematics and what it means to teach mathematics in school. (DDR)