Abstract: Differentiated instruction is necessary to meet the needs of diverse learners, particularly those in mixed ability classrooms (Gregory and Chapman, 2012; Sulla, 2013). This action research study explored one middle school that recently transitioned from ability grouped classrooms to mixed ability classrooms and prioritized differentiated instruction. Therefore, this study examined the teaching staff’s perceptions regarding the professional development they received in the area of differentiated instruction. Additionally, this study explored teacher perceptions about differentiated instruction and how well they were planning for its implementation. The study revealed the overall positive impact of professional development on teachers’ ability to differentiate instruction; that teachers vary on their core belief that all students are capable of learning within a mixed ability setting; that differentiated instruction is seen as a necessary instructional strategy, but is difficult to implement; and a direct correlation between teachers’ ability to differentiate instruction and the amount of choice they provide to their students.