The number of English language learners (ELLs) within the school system in one Western U.S. state continues to rise; writing scores of ELLs lag well behind those of their English speaking peers. The purpose of this ex post facto quantitative causal comparative study was to examine the writing achievement of fourth grade ELLs instructed within a transitional bilingual program and English immersion program. The conceptual framework for this study merged Krashen's monitor model, Cummins basic interpersonal communication skills and cognitive academic language proficiency, and Vygotsky's language and thought relationship model. The research question involved understanding differences between the writing achievements of students receiving instruction in their native language compared to students immediately mainstreamed into an all-English classroom. Writing test scores were collected from 68 students who participated in the two bilingual programs. The mean scores of these groups were compared using an independent t test. The findings indicated that there was no statistically significant difference between the two groups on writing achievement. This study contributes to social change because it is geared at improving ELL writing instruction, shifting the focus from language of instruction to effective writing instruction, and addressing the academic and linguistic needs of ELLs. Implications for social change also include on-going professional development for teachers in the area of ELL writing instruction, second language acquisition, and cultural competency. Teachers of ELLs as well as school districts with an ELL population will benefit from this as it will provide a better understanding of how language of instruction affects writing achievement of ELLs. [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.]