Effects of child and environmental factors in moderating the course of bilingual development were investigated using longitudinal data, from age 2.5 to 5 years, on 126 U.S.-born children with early exposure to Spanish and English. Multilevel models of Spanish and English expressive vocabulary identified children's phonological memory ability as a significant predictor of both outcomes, while also replicating the effect of the relative amount of language exposure. In addition, nonverbal IQ was a significant predictor of English vocabulary; birth order and maternal education in Spanish were significant predictors of Spanish vocabulary. These findings expand our understanding of the sources of the wide heterogeneity in bilingual development and of the requirements that language acquisition makes of learners and their environments.