This article presents a comparative analysis of five quasi-experimental studies involving close to 1,200 students, ranging in age from 7 to 14, in 49 French immersion classrooms in Canada – a content-based instructional context where learners develop high levels of communicative ability yet demonstrate a levelling-off effect in their grammatical development. The studies investigated the effects of form-focused instruction on four areas known to be difficult for anglophone learners of French: perfect vs. imperfect past tense, conditional mood, second-person pronouns and grammatical gender. Findings suggest that effective form-focused instruction in immersion contexts, at least with respect to interlanguage features that have reached a developmental plateau, includes a balanced distribution of opportunities for noticing, language awareness and controlled practice with feedback. Less effective instructional options overemphasise negotiation for meaning in oral tasks where message comprehensibility and communication strategies circumvent the need for learners to move beyond the use of interlanguage forms.