In this paper, we take a longitudinal case study approach to explore changes in two middle school teachers' beliefs and their self-reported changes in practice, as they implemented a cognitive apprenticeship intervention focused on teaching written scientific explanations. We collected interview data as the primary data source and triangulated it with the teachers' choices of tasks to explore how their beliefs about their students, their beliefs about writing, and their choices of instructional tasks changed over time. Findings suggest shifts in teachers' beliefs and changes in their instructional practices (i.e., choice of tasks) following implementation of the cognitive apprenticeship. Our study found that the utilization of cognitive apprenticeship influenced their beliefs and, consequently, their choice of writing tasks.