This pilot study sought to examine the mathematical knowledge for teaching that pre-service teachers used when participating in an online community, and to gain insight into their epistemological stance. The participants of this study were among the pre-service teachers in a large urban university, chosen as they were completing their mathematics methods course in their teacher-education program and before entering a field experience in the same academic year. A qualitative analysis of the online discussions of our participants was done using Ball and her colleagues' (2008) framework for mathematical knowledge for teaching and that of communities of practice (Wenger, 1998). These theories provided insight into the development of pre-service teachers as they moved from "student" to "teacher". Our findings show that pre-service teachers struggle to shed their student-perspective as they transition from theory to practice. This was readily evident in how they used their mathematical knowledge for teaching in their online exchange. Our work contributes to understanding the complexity of becoming a mathematics teacher in elementary school.