The hypothesis was tested that children whose families differ in socioeconomic status (SES) differ in their rates of productive vocabulary development because they have different language-learning experiences. Naturalistic interaction between 33 high-SES and 30 mid-SES mothers and their 2-year-old children was recorded at 2 time points 10 weeks apart. Transcripts of these interactions provided the basis for estimating the growth in children's productive vocabularies between the first and second visits and properties of maternal speech at the first visit. The high-SES children grew more than the mid-SES children in the size of their productive vocabularies. Properties of maternal speech that differed as a function of SES fully accounted for this difference. Implications of these findings for mechanisms of environmental influence on child development are discussed.Family socioeconomic status (SES) is a powerful predictor of many aspects of child development. An aim of current research is to identify the pathways by which SES exerts its well-established influence (DeGarmo, Forgatch, & Martinez, 1999;Keating & Hertzman, 1999;Linver, Brooks-Gunn, & Kohen, 2002; National Research Council and Institute of Medicine, 2000). Because SES and child development are multifaceted variables and because many factors that influence child development covary with SES, the causal relations underlying SES effects on child development may be difficult to uncover (Hoff, Laursen, & Tardif, 2002). The present study focused on one reliably observed relation between SES and child development and sought to identify the underlying mechanism.The relation in focus is that between SES and early vocabulary development. It is well established that children from lower SES build their vocabularies at slower rates than children from higher SES (Arriaga,