Studies by the National Association for Educational Progress found that English Language Learner (ELL) students perform poorly compared to other students on standardized mathematics exams. The research problem addressed how Sheltered Instruction Observation Protocol (SIOP) affected the instructional practices of high school mathematics teachers. The purpose of this evaluative case study approach was to explore the extent to which the implementation of SIOP influenced mathematics instruction in a mid-sized rural high school. The conceptual framework for this study was formed by combining Krashen's i+1 nativist theory for language acquisition through comprehensible input, Long's interactionist theory for acquisition of knowledge and Bandura's teacher efficacy theory. A concurrent mixed method design was selected to draw together inferences from both qualitative and quantitative data. NVivo software was used to combine a line by line analysis of interviews with an analysis of the components on the SIOP observation checklist. Interview findings suggest that teachers expressed a favorable response to SIOP since implementation. Classroom observations confirmed the efficacy of SIOP implementation. An analysis of covariance was used to evaluate mathematics achievement data from the Measurement of Academic Progress. Quantitative findings indicated no significant increase in mean scores after the first year of SIOP implementation. The results of this study could enhance the capacity of mathematics teachers to adjust instruction appropriate for their second-language development needs. Implications for positive social change include removing language barriers so that more ELL students may continue taking advanced mathematics courses and enter rewarding math-related careers. [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/en-US/products/dissertations/individuals.shtml.]