The paper describes an application of the XML technologies in the development of the BISIS library system. XML can be used in different applications in the development of library systems, for example, for description, storage and exchange of bibliographic data. In the BISIS system, XML is used for description of the bibliographic formats, as well as for description of bibliographic record structure. Besides, XML is used for description of the queries used for searching and downloading records from distant repositories. In order to enable circulation processes in a consortium, XML is used for data exchange related to the library users.