An ontology directory service tool, computer program product and method of automatically discovering ontology file categories. A web search unit searches a network (e.g., the Internet) for semantic data files, e.g., semantic web pages. A preprocessing unit generates an ontology file from the content of each identified semantic data file. A category discovery unit identifies a domain for each ontology file and provides training sets for training ontology file classification. A classification unit trained using the training sets, classifies ontology file instances into inherent ontology categories.