In a longitudinal study of 89 black children from different social classes, while there were no significant SES differences on the Cattell Infant Intelligence Scale at 18 and 24 months of age, there was a highly significant 23 point Mean IQ difference between children from welfare and middle class black families on the Stanford-Binet at 3 years of age. The range in Mean IQs of the black children in the extreme SES groups (93-116) was almost identical to that obtained by Terman and Merrill in their standardization sample of 831 white children between 2 1/2 and 5 years of age. The unique contribution of the present study is that the same pattern of social class differentiation in cognitive development, emerging during the third year of life, previously reported for white children has now been demonstrated for black children. (Author/MH)