Although immersion programs are very popular in Canada, they are complex to implement and not as successful as we would hope them to be when it comes to the development of students' language skills. This article reports on the findings of a case study conducted within the context of an ongoing five-year grant funded project in Western Canada whose overarching goal was increased student achievement through targeted professional development (PD) intervention. The specific intervention, a year and a half-long PD program based on an adapted integrated framework for curricular development (Cammarata, Laurent. 2016. "Foreign Language Education and the Development of Inquiry-driven Language programs: Key Challenges and Curricular Planning Strategies." In "Content-based Foreign Language Teaching: Curriculum and Pedagogy for Developing Advanced Thinking and Literacy Skills," edited by Laurent Cammarata, 123-143. New York: Routledge/Taylor Francis) and a Lesson Study model, was designed for 15 participating grades 6-12 French immersion teachers. It aimed at increasing their ability to craft and implement well-integrated instructional modules in order to better support students' linguistic and literacy skills development during subject-matter instruction. Data collected for this study exploring teachers' experience of appropriating and implementing the integrated approach included co-constructed lesson plans, classroom observations, focus-group debriefing sessions, and surveys. Findings indicate that long-term and sustained PD interventions organized around collaborative work can have a positive effect on teachers' ability to counterbalance content and language/literacy instruction. Important variables to ensure success are discussed and implications for immersion PD are shared.