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Description: Charlotte E. E. Wiltshire, Mark Chiew, Jennifer Chesters, Máiréad P. Healy & Kate E. Watkins Preprint available: https://psyarxiv.com/bhe57/ Abstract: Purpose: People who stutter (PWS) have more unstable speech motor systems than people who are typically fluent (PWTF). Here, we used real-time MRI of the vocal tract to assess variability and duration of movements of different articulators in PWS and PWTF during fluent speech production. Method: The vocal tracts of 28 adults with moderate to severe stuttering and 20 PWTF were scanned using MRI while repeating simple and complex pseudowords. Mid-sagittal images of the vocal tract from lips to larynx were reconstructed at 33.3 frames per second. For each participant, we measured the variability and duration of movements across multiple repetitions of the pseudowords in three selected articulators: the lips, tongue body, and velum. Results: PWS showed significantly greater speech movement variability than PWTF during fluent repetitions of pseudowords. The group difference was most evident for measurements of lip aperture, as reported previously, but here we report that movements of the tongue body and velum were also affected during the same utterances. Variability was highest in both PWS and PWTF for repetitions of the monosyllabic pseudowords and was not affected by phonological complexity. Speech movement variability was unrelated to stuttering severity with the PWS group. PWS also showed longer speech movement durations relative to PWTF for fluent repetitions of multisyllabic pseudowords and this group difference was even more evident when repeating the phonologically complex pseudowords. Conclusions: Using real-time MRI of the vocal tract, we found that PWS produced more variable movements than PWTF even during fluent productions of simple pseudowords. This indicates general, trait-level differences in the control of the articulators between PWS and PWTF.