Interest has long been regarded as an important motivational construct in the learning of mathematics. It has been contended that the development of interest is directed by two control systems: an emotional and a cognitive. Under the former, students are attracted to activities that are enjoyable, whereas under the latter they consciously engage in tasks that might satisfy, for example, later goals. Younger students are more likely to associate emotional responses to interest-related stimuli, whereas older students tend to associate cognitive responses, suggesting qualitative developmental differences for students' interest in mathematics. Yet there are differences between the classroom motivational environments experienced by primary/elementary vs. secondary school students, and the mathematical interest and enthusiasm likely felt and expressed by primary vs. secondary teachers, which may influence dimensions of student interest. Of concern in this study is the extent to which teacher enthusiasm impacts the classroom motivational environments (mastery vs. performance goal orientation) perceived by students and reported by teachers, and thereby dimensions of students' interest for mathematics. Using survey data from 471 students in Grades 3-10 and 44 of their mathematics teachers, we find that students' perceptions of their teachers' enthusiasm for teaching mathematics positively predict their perceptions of a classroom mastery environment, which in turn predicts their interest. However, at the class-level, teachers' enthusiasm for the subject of mathematics negatively predicts the emotional interest of their students. Implications of these results are discussed.