Peer argumentation, especially the discussion of contrary points of view, has experimentally been found to be effective in promoting science content knowledge, but how this occurs is still unknown. The available explanations are insufficient because they do not account for the evidence showing that gains in content knowledge are unrelated to group outcomes and are still evident weeks after collaboration occurs. The aim of this article is to contribute to the understanding of the relationship between peer-group argumentation and science content knowledge learning. A total of 187 students (aged 10 to 11 years) from 8 classrooms participated in the study, with the classrooms spread across 8 public schools, all located in Santiago, Chile. We conducted a quasi-experimental study randomized at school-class level. Four teachers delivered science lessons following a teaching program especially developed to foster dialogic and argumentative classroom talk (the intervention group), and four teachers delivered lessons in their usual way (the control group). Students were assessed individually using both immediate and delayed post-test measures of science content knowledge. The results showed no differences in pre- to post-immediate content knowledge between conditions. However, the intervention-group students increased their content knowledge significantly more than the control-group students between post-immediate and post-delayed tests. Hierarchical multiple regression analyses showed that, after controlling for school-level variables, time working in groups, and scores in the pretest, the formulation of counter arguments, although occurring in both groups, significantly predicted delayed gains in the intervention group only. Moreover, the frequency of counterarguments heard by students during the group work did not make a difference. Focal analysis of one small-group work suggests that teachers' instructional practice may have contributed to the consolidation of students' knowledge at an individual level in a post-collaborative phase.