This paper describes a small-scale study examining one facet of pre-service mathematics teachers’ knowledge for teaching. While mathematical knowledge for teaching has received extensive attention from researchers with regards to pre-service teachers, there has been little research on the mathematics register proficiency of this cohort. In this study, we examine a group of pre-service mathematics teachers’ mathematics register during a peer-teaching lesson. The authors adapt Rowland’s Knowledge Quartet as a framework for the study, by conceptually aligning mathematics register proficiency to each of the four dimensions: foundation, transformation, connection and contingency. Findings indicate pre-service mathematics teachers lack understanding of the significance of the mathematics register and its role in mathematics teaching for eliciting mathematical understanding for students. There was evidence of over reliance on the everyday register and a lack of fluency with regards to the mathematics register in practice. There is an exigent need for greater emphasis to be placed on developing preservice mathematics teachers’ mathematics register proficiency during initial teacher education, not only to improve pre-service teachers’ own knowledge and understanding, but also their ability to facilitate mathematics register proficiency in their future students.