The challenge that we address concerns teachers' shifts toward student-centered instruction. We report on a yearlong professional development study in which two United States elementary school teachers engaged in a teaching experiment, as described by Steffe and Thompson (in: Lesh and Kelly (eds) Research on design in mathematics and science education, 2000). The teaching experiment involved close mathematical interactions with a pair of students after school, in the context of solving fractions tasks. By conducting a teaching experiment, we anticipated that each teacher would have more opportunity to develop insight into students' mathematics. We also anticipated that these insights would influence the teachers' classroom practice, even without explicit support for such a shift. Indeed, the teachers found that they began asking more probing questions of their students and spending more time listening to students' explanations, but shifts to classroom practice were limited by constraining factors such an inflexible curriculum.