This paper examines acoustic aspects of vowel harmony (VH), understood as regressive vowel-to-vowel assimilation, in two regional varieties of French in six speakers' productions of 107 disyllabic word pairs. In each word pair, the word-initial vowel (V1) was phonemically either /e/ or /o/, and the word-final stressed vowel (V2) alternated between /e-E/, /ø-oe/, /o-O/ or /i-a/. Results are consistent with the idea that VH in French entails variations in tongue height along with related displacements of the tongue position along the front-back axis. These effects were independent of both the number of morphemes and lexical frequency. They were more systematic in Northern than in Southern French speakers' speech. Linear mixed-effects models strongly suggest that VH is a gradient effect of the trigger on the harmonizing vowel. Results lend support to usage-based phonological approaches regarding gradient phonetic differences as part of the gestural scores that make up the lexicon and that can be variably grammaticalized in different varieties of the language.