A series of Piagetian concrete operations period tasks dealing with classificatory concepts was administered to 280 children (40 subjects from each of seven levels--preschool, kindergarten, and first, second, third, fourth, and sixth grades). Significant main effects for age were found for all the tasks. Few significant sex differences were observed. Scaling analyses of the pass/fail patterns indicated a generally reliable (reproducibility coefficients of .91-.93) order of difficulty. Factor analysis of a subset of the tasks indicated that three principal components were necessary to account for 85 percent of the performance variance. Thus, a nonunitary structure was found for the children's performances across the present task array and age range. In general agreement with the original contentions of Inhelder and Piaget, it was concluded that the child's understanding of the logic inherent in class inclusion relationships evolves gradually and is contingent upon the previous mastery of certain less complex classificatory skills. (Author/DEP)