1. The gastric central pattern generator (CPG) driving the three teeth of the gastric mill inside the lobster stomach has often been used as a model for the study of central nervous systems, but the actual functioning of the mill has never been observed directly. By using a small endoscope inserted through the esophagus a video analysis of the tooth movements was performed with restrained, but otherwise intact lobsters. 2. The teeth show spontaneous periodic chewing (cycle duration from 4 to 70 s) in two different basic modes. In the squeeze mode only the cusps of the three teeth move together simultaneously. In the cut-and-grind mode the lateral teeth close first with not only their cusps, but also their serrated edges. After this cut phase the lateral teeth grind backward along the file of the medial tooth, which simultaneously moves forward. 3. Simultaneous endoscope recordings of the teeth, filming of stomach muscles and ossicles combined with electrical stimulation of selected muscles reveal that muscle gm3c is responsible for this hitherto unknown backward grinding of the lateral teeth. 4. The complete behavioral repertoire includes the following modifications of the two basic modes. 1) The lateral teeth can perform chewing movements while the medial tooth stays still and vice versa, forms of chewing regarded even weaker than the squeeze. 2) There do not appear to be intermediates between the squeeze and cut-and-grind movements, with the latter as the strongest form of chewing. Transitions only occur as switching on a cycle-by-cycle basis. 3) A gradual change of the cut-and-grind chewing was observed as the gradual development of an additional opening over the time course of several periods. 4) After their grind phase, the lateral teeth can even move further back beyond the medial tooth. This can serve to push food into the pyloric filter apparatus. 5. Inflation of the cardiac sac can elicit single bites in a resting gastric mill. 6. The behavioral repertoire is compared with the in vivo activity of the gastric oscillator represented by simultaneous intracellular recording from 7 representative cells of the 11 CPG neurons.