In an effort to isolate the emergence and causes of social class differences in intellectual performance, this longitudinal study was undertaken as a follow-up on a cross-sectional study that yielded no social class differences on the Cattell Infant Intelligence Scale for 12-, 18-, and 24-month-old black children. In the present study, 89 children from the 18 and 24 month samples of the previous study were tested on the Stanford-Binet at 3 years of age, and their mothers were given the Peabody Picture Vocabulary Test. There were highly significant differences on the Stanford-Binet between groups based on different socioeconomic status. Correlations between child's score and mother's score tend to increase with the child's age. These findings match those previously reported for white children. Interpretation of the data seems to indicate that social class influences on intellectual performance are operating but statistically insignificant at 18 and 24 months, finally becoming significant during the third year of life. Rather than being caused by either malnutrition or hereditary factors, social class differences in intellectual development may be due to differences in the acquisition of abstract knowledge, the pattern of verbal interaction between parents and child, and differences in symbolic thinking ability. (MH)