Preservice teachers (PSTs) are expected to possess a relational understanding (i.e., knowing how to do and why) of mathematics for ambitious instruction. This study aimed to shed some light on the possibilities of supporting PSTs' development of relational understanding of fractions through engaging them in writing collective argumentation. Drawing data from a larger project; we explored the development of a PST's understanding of fractions through the engagement of collective argumentation. The results indicated that the PST's relational understanding of fractions developed from both structural and content perspectives. Some educational implications for teacher education are discussed. [For the complete proceedings, see ED657822.]