This article reports findings from a study that investigated math achievement differences between English language learners (ELLs) and fully English proficient (FEP) students on a literacy-based performance assessment (LBPA). It has been assumed that LBPAs are superior to standardized multiple-choice assessments, but it has not been determined if LBPAs are appropriate for measuring the math achievement of ELLs. The most salient characteristic of LBPAs is that students read multi-level questions and explain how they solve math problems in writing. Thus, LBPAs place great literacy demands upon students. Because most ELLs have underdeveloped literacy skills in English, these demands put ELLs at a great disadvantage. Analysis revealed that socioeconomic status (SES) had a significant impact on all students, but the impact was larger on FEP students than on ELLs; high-SES FEP students outperformed high-SES ELLs, but there was no significant difference between low-SES ELLs and low-SES FEP students. High SES generally means more cognitive academic language proficiency, because of the influence of non-school factors such as the presence of a print-rich environment. High-SES ELLs did not do as well as high-SES FEP students because of a lack of academic English. The nature of the examination masked their true abilities. The finding of no difference between low-SES ELLs and low-SES FEP students, however, could be a result of the fact that neither group had the advantage of high cognitive academic language proficiency; the FEP students' only "advantage" was superior conversational English, of little use for performing academic tasks. This article concludes that LBPAs, together with the current assessment-driven accountability system, seriously undermine equal treatment for ELLs. (Contains 2 figures, 8 tables and 1 endnote.)