Building a dictation machine involves the management of linguistic knowledge. We are dealing here with the explicit integration of a phonological module in the automatic dictation machine MAUD, of which a first version has already been implemented [1]. MAUD is made up of three main modules: an acoustic-phonetic decoder, a lexical module and a syntactic module. Some of the problems at the lexical level can be addressed with the help of a phonological module. The one we have developped comprises a set of phonological rules taken from generative phonology [2] in order to transform phonetic forms into phonological ones. The most important problem is to formalize the rules in order to use them in speech recognition. The implementation of these rules leans on IRIT work [3] based on two original notions: multi-pronunciation groups (mpg's) and contextual phonological group (cpg's). Experimental results illustrate the impact of phonological knowledge in the overall recognition process of a dictation machine