Student participation in online discussion forums is associated with positive outcomes for student achievement and satisfaction, but research findings on the impact of class size and instructors' participation on student participation have been mixed. The present study analyzed the frequency of instructor and student posts in asynchronous discussion forums in 500 online courses to examine factors contributing to student participation. Results showed significant effects of both class size and amount of instructor participation, with a significant interaction between the two. In medium class sizes (with 15-30 students), amount of instructor participation did not predict the number of posts per student, but in smaller classes, significant differences in student participation were found depending on amount of instructor participation. Implications for fostering student participation in online discussion forums and interpreting research in this area are discussed.