In this study, framed within a sociocultural theory of mind, we explore the role of languaging in mediating between students' understanding of a grammatical concept and their written production of the forms related to that concept. The development of scientific concepts, in this case of the concept of voice in French, involves the use of language to mediate awareness and understanding of it, a process we call languaging. Using Vygotsky's distinction between scientific and spontaneous (everyday) concepts, we demonstrate the development of the grammatical concept of voice in French in which our two university-level participants progressed from no knowledge of voice to an emergent knowledge through languaging about a series of cards containing an explanation of the concept. The nature of this emergent knowledge was highlighted by the discrepancy in performance between written and oral modes of testing. Implications for teaching and testing are reviewed. (Contains 3 tables, 2 figures, 11 excerpts, and 10 notes.)