English language learner (ELL) mathematics students at a California middle school did not meet Adequate Yearly Progress (AYP) toward state standards in 2010. The purpose of this project study was to examine 6th and 7th grade mathematics teachers' perceptions regarding differentiated instruction for ELL students' specific academic needs and to reveal how these perceptions affected ELL students learning in mainstream classrooms. Hord's professional learning community (PLC) model grounded the conceptual framework for this study with a culture of continuous, shared inquiry based on collegial trust. The research question investigated teachers' perceptions regarding how PLC teachers are able to differentiate instruction for ELL students in mathematics classrooms. A case study design included transcripts from semistructured interviews, member checking, and PLC minutes. A purposeful, unique convenience sampling technique was used to recruit 4 mathematics teachers from both the 6th and 7th grade PLCs for a total of 8 interviews. The interview data were analyzed using descriptive coding and organized into categories. Themes emerged around teacher capacity, academic vocabulary, instructional challenges, PLC support, and outcomes. The findings revealed a need for a systematic method of providing professional development of differentiated instructional strategies. Implications for positive social change through this study may provide educators, administrators, and other researchers with instructional strategies that will improve ELL students' academic achievement, which is a positive outcome that also benefits the community. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]