Against the backdrop of mentor teachers' reasoning about practice, we seek to understand shifts in intern teachers' reasoning about practice during a year in which they take their final methods course in the fall and then do intern teaching in the spring. The data we analyze consist of intern and mentor teacher study group discussions of repeated viewings of an animation representing mathematics classroom practice around the solving of linear equations. Our analysis utilizes Toulmin's framework of argumentation and the construct of professional obligations from the practical rationality of mathematics teaching. The analysis suggests that, in reasoning about practice, in the fall, intern teachers emphasized teachers' obligations to mathematics as a discipline and to students as individuals, but as they moved into teaching, their arguments about practice include greater attention to obligations to the institutional goals of schools and to their class as a whole.