Understanding teachers' beliefs is important because beliefs influence teacher decisions. In science, teacher beliefs have an impact on how science curriculum is interpreted and implemented in the classroom. With the push for science, technology, engineering, and mathematics (STEM) education in the United States, it is also critical to examine the beliefs of teachers who integrate science in the classroom. This study of 21 U.S. middle school science and mathematics teachers found that teachers' participation in the first year of a two-year graduate online program that emphasised inquiry-based instruction and student-centred frames of mind influenced participants' beliefs. Overall, participants moved toward holding more student-centred beliefs. When types of beliefs were disaggregated, participants' beliefs about teaching and about learning both moved toward a more student-centred position. Further, teachers' beliefs significantly changed regardless of their years of teaching experience. One surprising finding was that science teachers' beliefs changed significantly, while those of mathematics teachers did not. The findings from this study support the notion that formal knowledge has an impact on teacher beliefs.