A number of experimental interventions have confirmed the effectiveness of peer collaboration for the acquisition of concepts such as conservation. However, no study to date has described the process or effectiveness of peer learning as it takes place in classrooms. The present study documented episodes of spontaneous peer interaction during academic tasks in a progressive school. Results indicated that the type of task performed influenced the structure of the interaction. Open-ended or discovery tasks such as story-writing promoted more exchange of information and more argumentation than did skill tasks that had only one right answer. Skill tasks were related to more negative behaviors, such as refusals and rejections, and resulted in more time spent off task. The interactive behavior observed during engagement with discovery tasks is consistent with the interactive style related to cognitive growth in laboratory studies of peer interaction.