Experimental evidence has shown the effect of peer-group argumentation on scientific concept development. However, questions regarding how and why it happens remain. The aim of this study is to contribute, with experimental evidence gathered in naturalistic settings (classrooms), to the understanding of the relationship between peer-group argumentation and content knowledge learning, exploring the role that individual argumentative skills play. In total, sixty-one fourth-grade students (aged 9-10 years) participated in the study (thirty-nine female). One teacher was invited to teach a thematic unit (Forces), with lesson plans especially developed to foster argumentation in the classroom. The second teacher taught "as usual." Students' conceptual understanding and argumentative skills were evaluated individually, both before and after the lessons. Although there were no differences in the immediate post-test scores between groups (after controlling for pre-test), the intervention group showed significantly higher scores in delayed post-tests. Regression analyses showed that the ratio of argumentative utterances per minute of group work predicted students' scores in delayed post-test disciplinary content knowledge after controlling for initial levels of learning. Argumentation skill gains did not impact learning, but initial levels of argumentation skills predicted delayed scientific content knowledge post-test.