The purpose of the present study was to investigate the difference of muscular activities of the tongue and lip muscles between anterior open-bite and normal occlusion during various function. Thirty eight subjects of 12.1-39.6 years were employed in this study: Eighteen subjects were anterior open-bites and twenty subjects were normal occlusions. During rest position, maximum effort of lip sealing, command swallow of saliva and sucking & swallowing of juice, the electromyographic datas of the genioglossus, superior longitudinal muscle of tongue, upper and lower orbicularis oris were recorded and integrated with Medelec MS 25 electromyographic machine. Lateral cephalometric radiographs were taken on all subjects for the analysis of tongue posture and size. All data were recorded and statistically processed. The findings of this study can be summerized as follows : 1. During the rest position, muscular activity of tongue was higher than that of lip muscle in open-bite, but no difference in normal occlusion group. 2. During the swallowing of juice, the activity of orbicularis oris was higher than that of genioglossus in open-bite. But the normal occusion was showed higher activity of tongue during the swallowing of juice. 3. The open-bite, compared to the normal occlusion, had higher value of tongue muscle activity during the rest position. 4. Compared to normal occlusion group, the open-bite had higher value of lip muscle activity during the sucking and swallowing of juice. 5. During the command swallow of saliva, the normal occlusion was showed harmonious swallowing pattern, but the open-bite showed much variation and disharmony in muscular contraction sequence, tended to absent of lip muscle activity. 6. At the lateral cephalometric radiographs, compared to the normal occlusion, the open-bite was showed lower tongue posture. But no difference was found in intermaxillary space. It is suggested that the open-bite, compared to the normal occlusion, had higher muscular activity of tongue during the rest position and various functions, showed lower tongue posture on the lateral cephalogram. In the development of anterior open-bite malocclusion, these factors may be an etiologic factor.