The purpose of this cluster randomized group study was to investigate the effect of multitiered, dual language instruction on children's oral language skills, including vocabulary, narrative retell, receptive and expressive language, and listening comprehension. Participants were 3-5 year old children (n = 81) who were learning English and whose home language was Spanish. Across the school year, classroom teachers in the treatment group delivered large group lessons in English to the whole class twice per week. For a Tier 2 intervention, teachers delivered small group lessons four days a week, alternating the language of intervention daily (Spanish, then English). Group post-test differences were statistically significant with moderate to large effect sizes favoring the treatment group on all English proximal measures and on three of the four Spanish proximal measures. Treatment group advantages were observed on Spanish and English norm-referenced standardized measures of language (except vocabulary), and a distal measure of language comprehension. [This paper was published in "AERA Open" v6 n1 p1-16 Jan-Mar 2020.]