Similar to other online courses, massive open online courses (MOOCs) often rely on learner-learner interaction as a mechanism to promote learning. However, little is known at present about learner-learner interaction in these nascent informal learning environments. While some studies have explored MOOC participant perceptions of learner-learner interactions, research is still lacking regarding the content and level of such interactions. Using the interaction analysis model (IAM) as a theoretical framework and social network analysis methods, the present study investigates the nature and level of learner-learner interaction within a popular Chemistry MOOC from Coursera. Findings suggest that learner-learner interaction: was limited to lower phases of the IAM framework (e.g., sharing and comparing information); changed (decreased) over time; and was heavily dependent on a few highly-engaged learners. Potential implications for the design of future MOOCs are discussed.