High school biology students (n = 48) were each randomly assigned to one of four groups defined by possible combinations of two teacher vagueness conditions (high vagueness versus low vagueness) and two post-lesson conditions (test, then lesson evaluation versus lesson evaluation, then test). Each group was presented a lesson over the vascular system of angiosperms. After the lesson, each group was tested on comprehension of the material and each group completed a lesson evaluation. Teacher vagueness terms significantly affected student achievement and student perception of lesson effectiveness. These findings are discussed in relation to previous research on teaching clearly.