Argumentation is a core practice of science and has recently been advocated as an essential goal of science education. Our research focuses on the discourse in urban high school science classrooms in which the teachers used the same global climate change curriculum. We analyzed transcripts from three teachers' classrooms examining both the argument structure as well the dialogic interactions between students. Between 19% and 35% of the discourse focused on scientific argumentation in that students were using evidence and reasoning to justify their claims. Yet in terms of dialogic interactions, only one teacher's classroom was characterized by student-to-student interactions and students explicitly supporting or refuting the ideas presented by their peers. This teacher's use of open questions appeared to encourage students to construct and justify their claims using both their scientific and everyday knowledge. Furthermore, her explicit connections to previous students' comments appeared to encourage students to consider multiple views, reflect on their thinking and reflect on the thinking of their classmates. This study suggests that a teacher's use of open-ended questions may play a key role in supporting students in argumentation in terms of both providing evidence and reasoning for students' claims and encouraging dialogic interactions between students. © 2009 Wiley Periodicals, Inc. Sci Ed94:203–229, 2010