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Data from the Children of the NLSY79 (CNLSY) are pooled together across survey waves, 1986-2000, to provide an unusually large sample size, as well as two or more observations at different time points for many children, recorded at single months of age between 36 and 156 months. We fit a variety of multilevel growth models to these data. We find that by 36 months of age, large net social class and Black-White vocabulary knowledge gaps have already emerged. By 60 months of age, when kindergarten typically begins, the Black-White vocabulary gap approximates the level it maintains through to 13 years of age. Net social class differences are also large at 36 months. For whites, these cease widening thereafter. For Blacks, they widen until 60 months of age, and then cease widening. We view these vocabulary differences as achieved outcomes, and find that they are only very partially explained by measures of the mother's vocabulary knowledge and home cognitive support. We conclude that stratification studies as well as program interventions should focus increased effort on caregiver behaviors that stimulate oral language development from birth through age three, when class and race gaps in vocabulary knowledge emerge and take on values close to their final forms. [Copyright 2003 Elsevier Inc. All rights reserved.]