This study examines the extent to which conceptual change is stimulated and achieved through online discussion in the context of an online graduate course. Transcripts of discussions among 15 graduate students studying assessment issues in mathematics and science education were analyzed using an interaction analysis model developed to assess knowledge construction and conceptual change in the context of an online debate. Although evidence of authentic conceptual change was limited, significant cognitive activity supporting conceptual change was identified. We offer observations about course characteristics that contributed to conceptual change, discuss the limitations of applying an existing content analysis model to a new context, and suggest further research focusing on mathematical discourse and scientific inquiry in the online environment. (Contains 2 tables and 1 figure.)