Argumentation is a core practice of science that inherently contains uncertainty. Relatively few studies have examined the role of uncertainty within argumentation and how teachers manage uncertainty leading to conceptual development. This design-based, multiple-case study employed the constant comparative method to analyze 24 videos focused on whole-class discussion, examining how two middle-school teachers created productive moments of uncertainty in an argumentative environment. Results showed that uncertainty in argumentation created productive moments for students to collaborate in dialogue and navigate their understanding of natural phenomena toward more coherent scientific explanations. Productively managing uncertainty was influenced by how the students' epistemic understanding of argument was used as a resource to create a space to engage in social negotiation. Creating productive moments of uncertainty involved the teachers (a) "raising" uncertainty about an authentic, meaningful, and ambiguous phenomenon; (b) "maintaining" uncertainty through seeking the flaws, incoherences, and inconsistencies of an argument; and (c) "reducing" uncertainty by synthesizing and bridging what students had learned with what they were learning. As a resource, the epistemic understanding of argument is intertwined with the practice of social negotiation and depends both on the students' degree of existing knowledge of dealing with uncertainty and the degree of their understanding of what counts as data, evidence, and reasoning.