In the design of a Multilingual Lexical Database, one of the biggest problems is constituted by conceptual mismatches between languages, and the resulting matter of lexical gaps. Lexical gaps concern words for which there is no direct translation in a target language, but which nonetheless need to receive a translation within the system. In this article, it will be shown that the various possible ways of dealing with these lexical gaps can be classified in four basic groups. Using the SIM<it>u</it>LLDA system as an example (Janssen 2002), the advantages of the structured interlingua approach over the other possibilities will be explained. With the SIM<it>u</it>LLDA set-up, it is possible to derive correct lexical definitions for lexical gaps from the lexical database. How this process of “lexical gap filling” works will be shown using a concrete example of a lexical gap: the treatment of the English words <it>river</it> and <it>stream </it>in contrast with the French words <it>fleuve</it> and <it>rivière</it>.