Knowledge-based systems have become ubiquitous in recent years. Knowledge-base developers need to be able to share and reuse knowledge bases that they build. Therefore, interoperability among different knowledgerepresentation systems is essential. The Open Knowledge-Base Connectivity protocol (OKBC) is a common query and construction interface for frame-based systems that facilitates this interoperability. Protégé-2000 is an OKBCcompatible knowledge-base-editing environment developed in our laboratory. We describe Protégé-2000 knowledge model that makes the import and export of knowledge bases from and to other knowledge-base servers easy. We discuss how the requirements of being a usable and configurable knowledge-acquisition tool affected our decisions in the knowledge-model design. Protégé-2000 also has a flexible metaclass architecture which provides configurable templates for new classes in the knowledge base. The use of metaclasses makes Protégé-2000 easily extensible and enables its use with other knowledge models. We demonstrate that we can resolve many of the differences between the knowledge models of Protégé-2000 and Resource Description Framework (RDF)-a system for annotating Web pages with knowledge elements-by defining a new metaclass set. Resolving the differences between the knowledge models in declarative way enables easy adaptation of Protégé-2000 as an editor for other knowledgerepresentation systems.