Objectives: This study examined the effectiveness of Sound Partners, an evidence-based early literacy intervention program, with culturally and linguistically diverse elementary students within an applied Response to Intervention (RTI) system. Method: Participants (n = 11) with a range of English language proficiency (ELP) who spoke English, Hmong, Khmer, or Nepali as a first language were recruited based on indicators of high academic risk displayed during the participating school's universal literacy screening process. Three replications of a randomized multiple baseline single case research study were conducted to assess the effectiveness of Sound Partners in three groups: those with lower ELP, middle ELP, and native English speakers. Progress monitoring of early literacy skills occurred weekly across baseline and intervention phases. Results: Visual analysis results supported two intervention effects: PSF effects in the low ELP replication and LSF effects in the native English speaker replication. Supplemental statistical analyses were conducted to describe the magnitudes and variations of effect sizes. Conclusion: Results suggest that key factors related to heterogeneity within the diverse population of ELL students, such as ELP levels, must be carefully considered when implementing RTI.