How language teachers talk is a key factor in organizing and facilitating learning specifically in classrooms where the medium of instruction is also the subject matter. This study aimed to examine the extent and ways of teacher talk adaptation to students’ proficiency levels in Iranian EFL context. Two EFL teachers who were teaching three different proficiency levels were observed and recorded. They were also interviewed to see if they made any conscious effort to adapt to their students’ level. Furthermore, the students of the same classes were interviewed for the comprehensibility of their teachers’ talk. Data were analyzed for four major areas of adaptation including speech rate, vocabulary, syntax, and discourse. The results showed that teachers’ adaptation to students’ level does happen clearly for elementary students; however, the adaptation boundary between the intermediate and advanced levels is fuzzy. The findings also indicated that in spite of what they think, teachers talk is not tuned to elementary students’ understanding.