Heterogeneity within English language learners (ELLs) groups has been documented. Previous research on differential item functioning (DIF) analyses suggests that accurate DIF detection rates are reduced greatly when groups are heterogeneous. In this simulation study, we investigated the effects of heterogeneity within linguistic (ELL) groups on the accuracy of DIF detection. Heterogeneity within such groups may occur for a myriad of reasons including differential lengths of time residing in English-speaking countries, degrees of exposure to English-speaking environments, and amounts of English instruction. Our findings revealed that at high levels of within-group heterogeneity, DIF detection is at the level of chance, implying that a large proportion of DIF items might remain undetected when assessing heterogeneous populations potentially leading to developing biased tests. Based on our findings, we urge test development organizations to consider heterogeneity within ELL and other heterogeneous focus groups in their routine DIF analyses.