Teaching presence facilitates students' social and cognitive presence in online courses. Instructor interaction in discussion forums, a widely adopted instructional strategy, establishes teaching presence but research on the optimal frequency and content of instructor interaction in discussion is underdeveloped. This research evaluated 1625 instructor posts in 36 graduate-level courses in education to determine their impact on students' perceptions of the quality of the instructor and course, students' perceptions of their learning, and students' actual achievement. Findings suggest the frequency of instructor interaction in discussion has no effect on student outcomes, but posts that are instructional improve students' perceptions of their learning, and posts that are conversational improve students' perceptions of instructor and course quality, and their actual academic achievement. Implications for instructors and policymakers are addressed.