The purpose of this research was to better understand the bilingual and biliteracy skills of Spanish-speaking low-socio-economic status (low-SES) children who attended an English or a bilingual programme during preschool and kindergarten/first grade, and to determine whether their outcomes varied according to instructional language and primary language proficiency. Subjects included 254 kindergarten through second grade (ages 4–7 years) Hispanic students who were low-SES, spoke Spanish as their primary language, and who had parents with low levels of formal education. Results showed that these children entered preschool with very low levels of language and literacy skills in Spanish and English, but made excellent gains regardless of the language instructional programme they attended. Children in English vs. bilingual instruction had significantly higher scores in Spanish and English language and literacy skills at preschool entry than children entering bilingual programmes, but these differences disappeared by first or second grade entry, and children instructed in English showed deteriorating Spanish language scores. Finally, among children instructed bilingually in the early years, those who were Mostly Proficient in Spanish at entry scored significantly higher on the English language proficiency test than those who were Mostly Limited in Spanish. Results provide evidence for the advantage of bilingual instruction at preschool through first grade levels for low-SES language minority children.