The aim of this work is to determine in French, through an analysis of the rate sensitivity of vowels and consonants, how the speech rate is related to the prosodic structure at the infrasyllabic level. We present an analysis of the phonemic duration of a one thousand word speech corpus produced at three rates by one speaker with two repetitions. Results show that in French consonants as well as vowels are more ratesensitive when stressed than when unstressed. This stronger rate sensitivity bears on all syllabic constituents. Still, consonants are less rate-sensitive than vowels and this is especially true for the stressed phonemes. We will account for the rate sensitivity of phonemes with a morphodynamic conception of phonology: where the acoustic substrate controls the phonemic form. We will see that the perception of speech rate is controlled by the same parameter which controls the features +/- stressed.