In this paper we describe the development of an ontology of molecular and phenotypic cereals data, realized by integrating existing public web databases with the database developed by the research group of the CEREALAB project1 . This integration is obtained using the MOMIS system (Mediator envirOnment for Multiple Information Sources), a mediator based data integration system developed by the Database Group of the University of Modena and Reggio Emilia2 . MOMIS performs information extraction and integration from both structured and semi-structured data sources in a semi-automatic way. Information integration is performed in a semi-automatic way, by exploiting the knowledge in a Common Thesaurus (defined by the framework) and the descriptions of source schemas with a combination of clustering and Description Logics techniques. The result of the integration process is a Global Virtual Schema (GVV) of the underlying data sources for which mapping rules and integrity constraints are spe...