Teachers and administrators in a Title I elementary school in a southeastern state are concerned that there has been a trend over the past 3 years of declining standardized assessment scores in mathematics for students in Grades 3, 4, and 5. The purpose of this qualitative case study was to explore teachers' perceptions of practices, and professional development (PD) for differentiating mathematics instruction. This study was grounded in the conceptual framework of Vygotsky's zone of proximal development and Tomlinson's differentiated instruction (DI). Purposeful sampling was used to select 8 teachers and 1 mathematics coach, who worked with students in Grades 3, 4, and 5 at the target school. Data were collected through interviews and classrooms observations with participants. Open coding and thematic analysis were used to identify emergent themes from the data. The key findings were that, while some teachers attempted to differentiate mathematics instruction for struggling students, the participants felt they needed more PD related to specific strategies for differentiation and more planning time to collaborate with other educators in order to implement differentiated instruction effectively for all students. In response to these findings, a PD project was created for teachers in Grades 3, 4, and 5 to involve them in construction of a variety of strategies for planning and implementing differentiated instruction in mathematics. Positive social change might occur when teachers feel supported by the district to schedule ongoing opportunities to collaboratively plan and implement effective differentiated mathematics instruction to advance students' mathematics achievement in the local Title I school.