This study attempted to prove a dichotomy between cognitive processes as measured by Piagetian reasoning tasks and those measured by traditional intelligence tests. A battery of tests (Slosson Intelligence Test, Durrell's Silent Reading and Listening Comprehension Test, Raven's Coloured Progressive Matrices, and the Piagetian Measure of Cognitive Development) was administered to 120 third-grade and fifth-grade children who were divided into groups according to race, sex, and social class. As predicted, test scores were clustered into two categories: one group, reflecting cognitive ability, consisted of scores from the Slosson and Durrell Silent Reading tests, while the other, reflecting cognitive development, consisted of scores from the Piagetian scales, the Raven test, and the Durrell Listening Comprehension measure. These results indicate that, in any attempt to assess the totality of a child's cognitive potential, both cognitive ability and cognitive development must be considered. The implications of these findings for lower-class and black children are discussed. (Author/KS)