This paper documents a study investigating co-construction of knowledge by doctoral students in an online learning community. In this study 12 students participated in the coursework and thesis proposal development stages of a doctoral program offered by a research-intensive university in New Zealand. Socio-cultural and social constructivist approaches were adopted to frame the design of this program. The findings of this study show that there was a high level of knowledge construction in some of the discussion forums of the program. Teaching strategies, in terms of how the learning tasks were designed, the amount of direct instruction provided, as well as how the discussions were moderated, were factors affecting knowledge construction. The active agency of the learners as well as the role assigned to them in the online discussion forums also had an impact on knowledge construction. This study also confirmed the importance of structure and leadership in online discussions.