A longitudinal, experimental-control design was used to test the hypothesis that native language instruction enhances English language learner's (ELL's) native language and literacy development without significant cost to English development. In this study, 31 Spanish-speaking preschoolers (aged 38-48 months) were randomly assigned to two Head Start classrooms differing only in the language of instruction (English and Spanish). As predicted, results showed that Spanish language instruction resulted in significantly higher growth on both Spanish oral vocabulary and letter-word identification measures. There were no significant differences between classrooms on these same measures in English. Results extend previous work by showing that Transitional Bilingual Education may be a viable alternative to traditional English-only models. Implications for theory, future research, and early childhood practice are discussed. (Contains 1 figure and 3 tables.)