The incidence and timing of activity in the jaw-closing muscles was investigated during speech. Electromyographic activity was analyzed from the masseter, anterior temporalis, medial pterygoid, and lateral pterygoid muscles in four normal adults. Jaw movement was transduced with a strain-gauge system as the subjects read the Rainbow Passage, 11 different syllables, and trains of the same syllables at four rates. Medial pterygoid was active for all jaw-closing movements. Masseter and temporalis were often active. The incidence of lateral peak electromyographic activity were noted between muscles; however, the duration of activity in medial pterygoid was longer than in masseter and temporalis. A large amount of variability in the relative degree of activity between muscles was observed for repetitions of the same syllable with similar parameters of jaw movements. These data are discussed in relation to series of muscle coordination.