Researchers examined teacher-related variation in the effects of a classroom intervention designed to impact seventh graders' beliefs about the nature of ability in science as fixed or malleable. Analyses of quantitative data from 7 science classrooms across 2 teachers revealed significant teacher effects in the extent to which students' beliefs about mindset, students' mastery-oriented learning goals, and students' achievement were sustained several months following the intervention. Classroom observational data and teacher reports of their beliefs offered some explanation for these differences. The teacher whose students had better outcomes placed more emphasis on mastery goals, growth mindset, conceptual development, and use of learning strategies in her daily interactions with students than did the other teacher. Program developers may want to design and study ways to impact teachers' practices in order to maximize and sustain program impact.