Abstract: There is a number of systems providing retrieval of multimedia documents. These systems have certain shortcomings, such as fixed document structure and a limited number of supported media types. An Extensible Multimedia Information Retri-eval System (XMIRS) has been developed as a result of addressing those shortcomings. This paper presents main characteristics of XMIRS software architecture providing system extensibility by multiple multimedia object retrieval modules and multiple document retrieval models. An implementation of a XMIRS prototype configured to support document retrieval in the Networked Digital Library of Theses and Dissertations (NDLTD) system is described in detail. Implementation of particular retrieval functionality utilizes existing software packages integrated into the whole system as XMIRS modules. The paper then presents examples of multimedia document retrieval in NDLTD using XMIRS including retrieval of text, images, and video clips.