The purpose of this study was to record the pattern of electromyographic (EMG) activity of supra- and infrahyoid muscles at different body positions and jaw posture tasks. The sample included 22 healthy subjects with natural dentition, bilateral molar support, and absence of posterior occlusal contacts during mandibular protrusion. Bipolar surface electrodes were located on the left supra- and infrahyoid muscles for EMG recordings. In each subject EMG activity was recorded while standing and in the lateral decubitus position, during the following jaw posture tasks: swallowing of saliva, maximal clenching in the intercuspal position, grinding from intercuspal position to protrusive edge-to-edge contact position, and grinding from retruded contact position to intercuspal position. Suprahyoid EMG activity was significantly higher in the intercuspal position than in all the other jaw positions studied (mixed model with unstructured covariance matrix). Suprahyoid EMG activity was similar in both body positions studied. Infrahyoid EMG activity in the intercuspal position was significantly higher than grinding from intercuspal position to protrusive edge-to-edge contact position, similar to swallowing of saliva, and significantly lower than grinding from the retruded contact position to intercuspal position. Infrahyoid EMG activity in the lateral decubitus position was significantly higher than in the standing position. The EMG pattern observed could be of clinical importance in the presence of parafunctional habits, i.e., clenching and/or grinding. The neurophysiological mechanisms involved are discussed.