Language difficulties of dyslexic subjects may result partly from a basic deficit in processing rapidly changing sensory inputs. In this fMRI study, we compared brain activities in adult dyslexics and controls during implicit categorical perception of phonemes with normal and slowed down stimuli. Perception of phonemic contrasts activated a frontal parietal network (Broca's area and the left supramarginal gyrus) in which the frontal component was down-regulated by slowed speech in controls and enhanced in dyslexic subjects. No modulation by speech rate was observed in the left supra-marginal gyrus. Enhancement of activity in Broca's area for slowed speech in dyslexic subjects might represent a neural basis of the improvement of performance that has been observed after remediation using this type of stimuli.