This paper examines the relationships between the elements of the Community of Inquiry (CoI) framework, disciplinary differences, perceived learning, instructor effectiveness, and delivery medium satisfaction. Specifically, the proposed research examines whether disciplinary differences such as those proposed by Biglan (1973a, 1973b) moderate the relationships between social, cognitive, and/or teaching presence and online course outcomes. Drawing from the results of a two-year study of students in over 50 online MBA courses, we found that disciplinary effects moderated the relationships between facilitating discourse, direct instruction and perceived student learning. Disciplinary effects did moderate the relationship between CoI elements and perceptions of instructor effectiveness. As disciplines moved closer to "pure" or "hard" status, social presence became positively associated and cognitive presence became negatively associated with perceived instructor effectiveness. (Contains 8 tables and 1 figure.)