Open-ended interviews were used to study differences in Grade 7-8 teachers' beliefs about exemplary classroom practices produced by exposure to in-service professional development programs based on different theoretical frameworks. Teachers who had participated in an intensive, inquiry-based, in-service professional development program that focused on preferred teaching methodologies from a constructivist, inquiry-based perspective described in the "best practices" literature were compared with teachers who had been through traditional in-service professional development workshops. Analysis of interviews based on the teachers' responses to both conceptual and algebraic questions that involved the concept of density revealed insight into the teachers' preferred teaching methodologies, their beliefs about how students learn, their attitudes toward their own learning, and their depth of understanding of the concept of density. This study suggests that, for some teachers, extensive in-service professional development can produce a substantive change in teachers' beliefs about optimum teaching practice. But this change can have unforseen consequences; in this case, the formation of a set of beliefs or values that forgot the importance of building quantitative skills as well a conceptual understanding of the concepts being taught.