Since the late 1990s, there has been consensus among educational researchers that argumentation should play a central role in science education. Although there has been extensive relevant research, it is not clear enough how oral argumentation spontaneously occurs in science teaching. This is particularly important with regard to the empirical evidence suggesting the effect of discussion of contradictory views on scientific learning. In order to contribute to the research on argumentation in science teaching, we conducted a study that aims to sketch a panoramic view of the uses of oral argumentation in Chilean middle-school science teaching. A total of 153 videotaped science lessons were observed, involving students aged 10-11 and 12-13. Whole-class argumentative discourse was analysed as a function of thematic episodes and teachers' and students' utterances. Results suggest that argumentative discourse in which contradictory points of view are discussed is scarce but when it occurs it does so predominantly within discourse among students. On the contrary, argumentation aimed at justifying points of view is widely used, even more so when students are older.