We report an analysis of the language used by two instructors teaching two undergraduate mathematics classes that exhibited high student participation yet differed in their level of dialogical engagement. We focus on the way instructors' language contributes to opening or closing the opportunities for students' engagement with mathematical dialog, in turn establishing parameters for students' agency in the classroom discourse. We found ways in which instructors used language to determine different student engagement. How instructors use their linguistic resources in engaging students may facilitate or forestall dialogic possibilities, which in turn can influence students' performance. (Contains 4 figures and 3 tables.)