The purpose of this study was to examine the developmental change in ability to understand hierarchical concepts, particularly to classify one object at two different levels simultaneously and to reason on the hierarchical relation between the two concepts. First, fourth and sixth grade children learned the artificial concepts of imaginary animals which were organized at two different levels. Information on the hierarchical relation concerning one group of animals, was given to half of the subjects. After the learning session children were asked about the names of the animals: first grade children could not apply both names of the different levels to one animal even after being given the information; on the other hand, fourth grade children could apply them to one animal. Sixth grade children could apply them to one animal though they were not given any information. The results were interpreted as evidence of a developmental change in children's abilities to understand hierarchical concepts.