Bullfrog tadpoles (above ca. 18 C) normally achieve branchial ventilation by the rhythmic pumping of water through the buccal cavity. This is facilitated by a coordination between the mandibular and hyoidean movements. Below 70 cycle/min only the lower jaw is active, but both jaws participate at higher frequencies. The jaw and hyoidean muscles and their time sequence were investigated by direct observation, electrical stimulation, lesion, and electromyography. A consideration of jaw ligaments has led to a new interpretation of the jaw mechanism.