In this study, we analyse a 9-year-old child's argumentative participation in a group discussion, from a microgenetic development angle. Specifically, we follow one child through the discussion and analyse the development process, defined as changed participation and as interactionally contingent upon the other participants and the teacher. The data consist of one video recording of the group discussion including two dilemmatic tasks for which the children are asked to come to a joint solution. A sequential analysis was performed from a sociocultural perspective on learning and communication. The findings show that the focus child's argumentative participation develops both quantitatively and qualitatively, and that this development reflects a switch between general agreement amongst the children in the first task and general disagreement in the second task. This disagreement angle is discussed both from an argumentation and an education perspective. Two of the focus child's argumentative resources (an imaginary space she creates and the use of the concept "data protection") are discussed in detail.