The purpose of this study was to investigate the way in which items on the Woodcock-Muñoz Language Survey Revised (WMLS-R) Spanish and English versions function for bilingual children from different ethnic subgroups who speak different dialects of Spanish. Using data from a sample of 324 bilingual Hispanic families and their children living on the United States mainland, differential item functioning (DIF) was conducted to determine if test items in English and Spanish functioned differently for Mexican, Cuban, and Puerto Rican bilingual children. Data on child and parent language characteristics and children's scores on Picture Vocabulary and Story Recall subtests in English and Spanish were collected. DIF was not detected for items on the Spanish subtests. Results revealed that some items on English subtests displayed statistically and practically significant DIF. The findings indicate that there are differences in the difficulty level of WMLS-R English-form test items depending on the examinees' ethnic subgroup membership. This outcome suggests that test developers need to be mindful of potential differences in performance based on ethnic subgroup and dialect when developing standardized language assessments that may be administered to bilingual students.