Many English language learners (ELLs) and children living in poverty begin school with substantially less English vocabulary knowledge than their monolingual, economically advantaged peers. Without effective intervention, these vocabulary gaps are likely to contribute to long-term reading failure. This quasi-experimental study examined the extent to which a family literacy program (FLP) moderated vocabulary development of 158 ELLs (prekindergarten through third grade) from low-income families in relation to children's level of vocabulary knowledge (at, moderately below, or substantially below national norms) on program entry. The FLP activities focused on supporting parents' development of English literacy and on teaching them effective ways to engage their children in authentic, home-based literacy events and practices that could be expected to prepare their children for success in school. Findings indicated that although all children demonstrated substantial language and literacy growth, children with the lowest pretest vocabulary knowledge achieved the greatest vocabulary gains and these gains differed significantly from their demographically matched peers who did not participate in the FLP. Gains for treatment and control children with middle and high pretest vocabulary knowledge (relative to this sample) did not differ significantly. Findings suggest that an FLP emphasizing authentic literacy practices holds particular promise in closing vocabulary gaps among children who enter early childhood classrooms with especially limited English vocabulary knowledge.