Ausonius' poetry is marked by a great deal of formal research, which takes five main aspects: metrical virtuosity, Latin-Greek bilingualism, centon, games on words, lexical elaboration. It is this last point that will hold our attention here. If the language of Ausonius is generally consistent with the classi- cal standards from the point of view of the syntax, it is not exactly the same for the lexicon. Ausonius sometimes uses late words and/or is influenced by the vulgar language. This characteristic of his language has been little studied and we must often resort to the old thesis of A. Delachaux. In this paper, we will review these late and/or vulgar words, to try to draw up a typology. We will then examine the sty- listic use that Ausonius makes of them, because, as a refined poet, he never chooses his words randomly. We will finally see if it is possible to infer some more general conclusions about the linguistic situation in Aquitania in the fourth century.