This article reports on an international study of the teaching of undergraduate mathematics in seven countries. Informed by rhetorical genre theory, activity theory, and the notion of Communities of Practice, this study explores a pedagogical genre at play in university mathematics lecture classrooms. The genre is mediational in that it is a tool employed in the activity of teaching. The data consist of audio/video-recorded lectures, observational notes, semistructured interviews, and written artifacts collected from 50 participants who differed in linguistic, cultural, and educational backgrounds; teaching experience; and languages of instruction. The study suggests that "chalk talk," namely, writing out a mathematical narrative on the board while talking aloud, is the central pedagogical genre of the undergraduate mathematics lecture classroom. Pervasive pedagogical genres, like chalk talk, which develop within global disciplinary communities of practice, appear to override local differences across contexts of instruction. Better understanding these genres may lead to new insights regarding academic literacies and teaching. (Contains 2 figures, 2 tables, and 5 notes.)