While coaching is a well-documented form of professional development to support teachers' instructional practices in mathematics, limited research exists on how coaches can support teachers in differentiating their mathematics instruction. Hence, as part of this retrospective case study, we explored how one instructional coach leveraged a coaching cycle to help an experienced elementary teacher differentiate her mathematics instruction for students. Specifically, the study sought to explore: (a) how the coach and teacher engaged in differentiating instruction, (b) what opportunities the coach and teacher had to discuss differentiation, and (c) what challenges the coach and teacher encountered. Data sources include observations of two planning meetings, four co-taught lessons, and two reflection conversations, as well as eight semi-structured interviews with the coach, teacher, principal, and district-level administrator. Results indicated that the coach and teacher primarily engaged in conversations about differentiating content and process, while other differentiation dimensions (e.g. product and learning environment) were not the focus. The same trend was observed during instruction. Additionally, the depth of the coaching differentiation conversation was primarily low to medium-level, and common barriers existed for the coach-teacher dyad. Implications for researchers and practitioners are provided.