The passage of Proposition 203 in Arizona in November 2000, virtually replaced bilingual education with a default program for English Language Learners--Structured English Immersion (SEI). The requirement is for nearly all instruction to be in English, with a minimal amount of the native language allowed. This mixed-method study chronicles the implementation of Proposition 203 in a local school district and examines its effects on instructional practices, student achievement, as well as on school climate and culture. Also described are the mitigating requirements of NCLB and Arizona Learns and their effect on instruction for ELL students.Eight teachers in grades K-3 in both SEI and bilingual education programs, and two elementary bilingual special education teachers participated in the study. Six of the ten total participants hold an endorsement in bilingual education. Student achievement data included an analysis of AIMS scores in reading, writing, and math for 2005 and 2006. Qualitative research methodologies were used to obtain classroom observation data. Teacher interviews consisted of open-ended questions related to teachers' understanding about Proposition 203 and its effects on their instruction and school climate.This study suggests that SEI has not been successful in raising student achievement and English proficiency to the levels its proponents had promised. High-stakes testing and other requirements of NCLB and Arizona Learns have exacerbated district attempts to expand bilingual education programs. The study concludes with a summary of continued challenges regarding effective ELL instruction and recommendations and proposed solutions from the literature and the field.