Despite the numerous studies on social interaction in collaborative learning, little is known about interaction forms in successful computer-supported collaborative learning situations. The purpose of this study was to explore and understand student interaction in successful collaborative learning during a university course which was mediated by two different types of virtual learning environment. Through a qualitative case study, we examined how students interacted with each other while working with collaborative tasks. Results indicate that interaction in collaborative situations was more often group-related than task-related. Group-related interaction concentrated mostly on coordination of group work, such as planning and organising group activities. Task-related interaction was mostly in the form of comments or answers to earlier messages. However, there were differences in the interaction forms according to the learning environment. The results of this study provide teachers, educators and educational coordinators guidelines for how to organise and enhance successful collaborative learning both virtually and face-to-face.