With the growing significance of digital libraries and the Internet, more and more electronic texts become accessible to a wide and geographically disperse public. This requires adequate tools to facilitate indexing, storage and retrieval of documents written in different languages. We present a method for semi-automatic indexing of electronic documents and construction of a multilingual thesaurus, which can be used for query formulation and information retrieval. We use special dictionaries and user interaction in order to solve ambiguities and find adequate canonical terms in the language and an adequate abstract language-independent term. The abstract thesaurus is updated incrementally by new indexed documents and is used to search for documents using adequate terms.