Determining the effectiveness of core mathematics curricula is foundational to evidence-based practice. Examining effectiveness heterogeneity is also crucial to better understanding mathematics achievement among English language learners (ELLs). In this study, we used a quasi-experimental design (difference-in-differences) to examine the impact of a standards-based elementary mathematics curriculum ("Bridges in Mathematics") on fifth graders' annual gains in mathematics achievement in a large midwestern school district compared to the district's prior curriculum ("Investigations"). We also investigated whether the effect of "Bridges" varied across English language proficiency (ELP) levels of ELLs. Students in schools that implemented "Bridges" (n = 1,839) showed significantly greater mathematics gains compared to those receiving the prior curriculum (n = 3,354; g = 0.25 in change score standard deviations). This effect did not vary significantly across ELP levels. Limitations of this study as well as implications for research and practice with core curricula are discussed. [For the corresponding grantee submission, see ED616693.]