This article investigates teacher enthusiasm and how it relates to instructional behaviours. We distinguished teachers' enthusiasm for the subject matter of mathematics from their enthusiasm for teaching mathematics. A total of 323 teachers and their 9th-grade classes participated in the study. Questionnaires were used to assess teachers' enthusiasm and instructional behaviours from both the teacher and the student perspective. Structural equation modelling revealed that teachers who were more enthusiastic about teaching showed higher quality instructional behaviour—both self-reported and student-rated. By contrast, enthusiasm for mathematics as a subject matter predicted teachers' self-reports, but not students' ratings, of instructional behaviour.