This study investigated the development of the understanding of class inclusion in children age 5, 7, and 9 years, whose performance on a qualitative class-inference task assessing their appreciation of the transitive and asymmetrical nature of inclusive relations within the animal domain was compared with their ability to make quantitative inferences in Piagetian class-inclusion problems. Results showed that, although 5-year-olds demonstrate a fair knowledge of the transitivity of inclusion relations, this notion is not fully understood until the age of 7. In contrast, the process leading to the acquisition of asymmetry understanding appears relatively slowly, and is not yet completed by 9 years of age. Whereas the ability to make qualitative inferences requiring the understanding of transitivity is acquired well before the ability to make quantitative inferences, making qualitative inferences requiring a knowledge of asymmetry is as difficult as making quantified judgments. Methodological considerations about the complementarity of the two kinds of tasks, along with the theoretical implications of our findings for Blewitt's developmental model of hierarchical knowledge, are discussed. (Contains 3 tables.)