A key issue in knowledge representation is the representation of hierarchies of concepts. Conceptual Graphs is particularly well suited to this problem, specially when structured descriptions are required for concepts. Descriptions of concepts are represented by conceptual graphs which are used to define types representing the concepts. Hierarchies of concepts are then represented by Aristotelian hierarchies defined on these types. However, Conceptual Graphs formalism becomes inadequate for the representation of concept hierarchies when exceptions occur in property inheritance between concepts. Other logics formalisms also deal with concept hierarchies while offering ways to account for exceptions but they are less intuitive and readable than conceptual graphs. Moreover, conceptual graphs are often used in knowledge representation. Therefore, we propose to extend Conceptual Graphs formalism in order to allow the representation of default taxonomic knowledge. We first show that the classical “default inheritance” approach appears to be inadequate for Conceptual Graphs formalism. We then adopt a “definitional” approach similar to the one pioneered by Coupey and Fouqueré in ALN terminological language where defaults and exceptions are seen as parts of the descriptions of concepts.