Abstract : This paper presents a thesaurus of ancient and medieval zoological knowledge, called THEZOO, constructed in the framework of the International Research Group Zoomathia. It aims at integrating heterogeneous data sources on zoology in Antiquity and Middle Ages: mainly texts, but also images, archaeological objects and archaeozoological material. The development process of THEZOO combines 1) the manual annotation of books VIII-XI of Pliny the Elder’s Natural History, chosen as a reference dataset to elicit the concepts to be integrated in the thesaurus, and 2) the definition and hierarchical organization of the elicited concepts in the thesaurus. THEZOO is formalized in SKOS, the W3C standard to represent knowledge organization systems on the Web of data, and it is created with the Opentheso editor. Our final aim is to publish the thesaurus THEZOO as well as the corpus of annotated texts, to support a semantic search in the corpus in different languages.