Psychometricians and test developers evaluate standardized tests for potential bias against groups of test-takers by using differential item functioning (DIF). English language learners (ELLs) are a diverse group of students whose native language is not English. While they are still learning the English language, they must take their standardized tests for their school subjects, including science, in English. In this study, linguistic complexity was examined as a possible source of DIF that may result in test scores that confound science knowledge with a lack of English proficiency among ELLs. Two years of fifth-grade state science tests were analyzed for evidence of DIF using two DIF methods, Simultaneous Item Bias Test (SIBTest) and logistic regression. The tests presented a unique challenge in that the test items were grouped together into testlets--groups of items referring to a scientific scenario to measure knowledge of different science content or skills. Very large samples of 10,256 students in 2006 and 13,571 students in 2007 were examined. Half of each sample was composed of Spanish-speaking ELLs; the balance was comprised of native English speakers. The two DIF methods were in agreement about the items that favored non-ELLs and the items that favored ELLs. Logistic regression effect sizes were all negligible, while SIBTest flagged items with low to high DIF. A decrease in socioeconomic status and Spanish-speaking ELL diversity may have led to inconsistent SIBTest effect sizes for items used in both testing years. The DIF results for the testlets suggested that ELLs lacked sufficient opportunity to learn science content. The DIF results further suggest that those constructed response test items requiring the student to draw a conclusion about a scientific investigation or to plan a new investigation tended to favor 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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]