© 2019, Western Australian Institute for Educational Research Inc.. All rights reserved. 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 pre-service 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.