Drawing on longitudinal data, this study explores the dynamic relationship between the development of conceptual knowledge of language and the emergence of sociostylistic variation in second language (L2) speech. Participants include eight intermediate-level US university learners of French who participated in a concept-based pedagogical enrichment program over the course of two months. After presenting an overview of the study's design, this article analyses the co-development of conceptual knowledge and one type of sociostylistic variation in French: the variable presence versus absence of the proclitic negative particle ne of verbal negation. The analysis centers on learners' orientations to scenarios to be performed as spoken-interactive tasks, including reasons for choosing different negative structures, their use of negative structures in actual performance, and the emergence of mediated and independent performance abilities. (Contains 5 tables and 2 figures.)