Jaw movements and electromyographs of the masseter and digastric muscles were recorded during mastication in the normal and osteopetrotic (op/op) mice. A masticatory sequence of the mice was divided into two stages dependent on the jaw movements: stage I and stage II. Small and rapid (8 Hz) open-close jaw movements were observed in the stage I, while slow (5 Hz) and large open-close jaw movements were observed in the stage II. The stage I and stage II are considered as incision and molar chewing stage, respectively. Rhythmic masticatory cycles of the op/op mice, in which tooth eruption does not occur, were similar to those of the normal mice. Although the duration of the stage I in the op/op mouse was significantly longer than that in the normal mouse, the cycle duration of the former was not significantly different from that of the latter. These results suggest that the central pattern generator (CPG) producing the chewing rhythm develops even without tooth eruption.