Construction of ontologies in specific domains (e.g., molecular biology, electronic commerce), is invariably a collaborative activity that requires incorporation of independently generated ontology fragments or ontology modules, and hence reconciliation of inconsistencies among ontology mod- ules. We investigate an approch to reconciling the inconsistencies among ontologies using defeasible axioms. In this framework, each ontology module can be viewed as an internally consistent unit (called a package) with well-specified access interfaces. Multiple ontology modules or packages can be combined to obtain larger ontologies. Inconsistencies between ontology modules are handled using defeasible axioms (an axiom in one package can defeat one or more axioms from other packages), thereby making the resulting composite ontology internally consistent. The re- sulting framework supports collaborative ontology construction as well as integration of preexisting ontologies.