Argumentation, as a kind of classroom discourse, is beneficial for establishing mathematical knowledge taking place in classroom conversations. It can enhance learners' development of subject matter knowledge. Hence, this case study was designed to examine the effect of argumentations on developing subject matter knowledge in detail in mathematics. For this purpose, discussions happened in a collective learning environment, designed based on problem-based learning and taking place in a six-week instructional sequence, was investigated focusing on argumentations. 23 preservice middle school mathematics teachers (PMSMT) engaged in tasks designed by the researchers through classroom interaction between them and the instructor in this environment. The PMSMT's mathematical ideas identified through Toulmin's argumentation model were documented in this study. With the help of this model, the mathematical ideas were extracted and the development of PMSMT's subject matter knowledge was analyzed and documented in detail. It was observed that the PMSMT improved their subject matter knowledge by forming mathematical ideas. They constructed new knowledge and revised their previous knowledge through argumentations.