When measuring academic skills among students whose primary language is not English, standardized assessments are often provided in languages other than English. The degree to which alternate-language test translations yield unbiased, equitable assessment must be evaluated; however, traditional methods of investigating measurement equivalence are susceptible to confounding group differences. The primary purposes of this study were to investigate differential item functioning (DIF) and item bias across Spanish and English forms of an assessment of early mathematics skills. Secondary purposes were to investigate the presence of selection bias and demonstrate a novel approach for investigating DIF that uses a regression discontinuity design framework to control for selection bias. Data were drawn from 1,750 Spanish-speaking Kindergarteners participating in the Early Childhood Longitudinal Study, Kindergarten Class of 1998–1999, who were administered either the Spanish or English version of the mathematics assessment based on their performance on an English language screening measure. Evidence of selection bias—differences between groups in SES, age, approaches to learning, self-control, social interaction, country of birth, childcare, household composition and number in the home, books in the home, and parent involvement—highlighted limitations of a traditional approach for investigating DIF that only controlled for ability. When controlling for selection bias, only 11% of items displayed DIF, and subsequent examination of item content did not suggest item bias. Results provide evidence that the Spanish translation of the ECLS-K mathematics assessment is an equitable and unbiased assessment accommodation for young dual language learners. (author abstract)