This article presents a case study that investigated primary school students’ social interaction while working in small groups in science. The aim was to identify what characterizes and triggers students’ efficacious interaction in collaborative learning situations. This was done by exploring and analyzing students’ and groups’ task involvement and the quality of their activity. The micro-level analysis proceeded first by identifying episodes of group task involvement and evaluating the quality of these interaction episodes. Next, the transitions between episodes were explored by identifying what triggered the shifts between task involvement levels. Ten hours of video observation data captured fourth-grade students, aged 9 to 10, in two groups of 3 students studying “the vital conditions of life” for 5 weeks. The main findings indicated that efficacious interaction demanded collaboration between group members and required active participation and productive on-task working. Three types of triggers were found to cause changes in students’ activity, of which the group progress trigger was the most powerful for increasing, and the contextual trigger the most powerful for decreasing the activity. These findings shed light on certain aspects of efficacious interaction that will help us to identify details of efficacious interaction in future studies.