Schema matching is a fundamental issue in many database applications, such as query mediation and data warehousing. It becomes a challenge when different vocabularies are used to refer to the same real-world concepts. In this context, a convenient approach, sometimes called extensional, instance based or semantic, is to detect how the same real world objects are represented in different databases and to use the information thus obtained to match the schemas. This paper describes an instance-based schema matching technique for an OWL (Web Ontology Language) dialect.