This study examined low-income, Spanish-speaking, immigrant Latina mothers' book sharing behaviors in relation to their children's vocabulary. Participants were 47 3-year-old children and their mothers. We addressed two research questions: (a) What interactive behaviors are evident when low-income immigrant Latina mothers and their 3-year-old children look at books together? (b) For these children and their mothers, which book-sharing behaviors are related to children's expressive language? Overall, our results indicated that mothers were involved in several kinds of interactions with the books. They enhanced their children's attention to the printed text, promoted interaction or conversation with their children about what was in the books, and somewhat less often, used more complex literacy strategies. Mothers who did these things most had children with the largest vocabularies even when mothers' vocabulary was taken into account. Implications for designing interventions for similar families are discussed.