This study is about graduate students’ discourse practices in a classroom text-based synchronous computer-mediated discussion (SCMD). Cultural historical activity theory (in short, Activity Theory) is the primary theoretical lens through which the data are analyzed. Engeström’s (1987) Activity System model among the various theoretical positions or perspectives of activity theorists has guided the overall process of the study, especially having the researcher focus on the identification and description of the model’s six key elements: subject, object, tool, community, rule, and division of labor. Several emerging themes were identified. An activity system in SCMD is situated in multiple dimensions of context: physical/biological, cultural/institutional, social/ emotional, and cognitive/intellectual dimensions; instead of a single utterance, a topical pair needs to be investigated as a unit of analysis in SCMD research; a collective unit of actions emerges through the discourse activity; and, finally, an ecological view is needed to understand an activity system as a whole. Based on these emerging themes, I conclude with a modified model of the activity system in the situation of dialogical transactions such as SCMD. / text