In this study, the authors examined the effect of group size on students' behavior in asynchronous, nonmandatory instructional discussion groups. The focus in this study is on four main questions: (a) Does group size affect the proportion of learner-learner and instructor-learner interactions? (b) Does group size influence the number of messages instructors post? (c) Does group size have an effect on the number of contributions that students post? (d) Does group size affect instructor lag time as well as the interval between successive postings? It was found that group size affects all these aspects of asynchronous discussion: The proportion of learner-learner interaction increased as group size increased, while the proportion of instructor messages decreased. Most students participated to a minimal degree and only a small minority of students posted more than 10% of a discussion group's messages. Last, as group size increased, the interval between successive instructors' postings decreased; lag time, however, was not influenced by group size. Results are discussed in terms of the restructured theory of transactional distance by Gorsky & Caspi, 2003, and several explanations relating group size and students' behavior in asynchronous discussion group are suggested.