French expressions like FRUIT DE MER 'seafood', MONTER UN BATEAU [a qqn] 'to fool someone' and A PROPOS [de qqch] 'about something' are idioms. Set phrases like these are considered in Meaning-Text Theory (MTT) as lexical units and are therefore described by an autonomous lexical entry in the dictionary. However, since each phrase consists of more than one word-form, these lexical units display complex behavior which has not yet been rigorously described in the Meaning-Text lexicography. In this paper, we present a MTT treatment of idioms focusing on their presentation in the dictionary, and propose tools for constructing a lexical database of idioms, which can efficiently represent form flexibility and combinatorial particularities of French idioms.