This study investigates speech motor control in 4-year-old Canadian French children in comparison with adults. It focuses on measures of token-to-token variability in the production of isolated vowels and on anticipatory extra-syllabic coarticulation within V 1-C-V 2 sequences. Acoustic and ultrasound articulatory data were recorded. Acoustic data from 20 children and 10 adults have been analyzed. Thus far, ultrasound data have been analyzed from a subset of these participants: 6 children and 2 adults. In agreement with former studies, token-to-token variability was greater in children than in adults. Strong anticipation of V 2 in V 1 was found in all adults, but not in children. Most of the children showed no anticipation at all and some of them showed a small amount of anticipation along the antero-posterior dimension only, manifested in the acoustic F2 dimension. These results are interpreted as evidence for the immaturity of children's speech motor control from two perspectives: insufficiently stable motor control patterns for vowel production, and a lack of effectiveness in anticipating forthcoming gestures. In line with theories of optimal motor control, anticipatory coarticulation is assumed to be based on the use of internal models of the speech apparatus and the increasing maturation of these representations as speech develops.