Introduction: In this prospective clinical study, we investigated the effects of rapid maxillary expansion ( RME) on the electromyographic ( EMG) activities of the anterior temporal and superficial masseter muscles. Methods: The sample included 18 subjects ( 11 girls, 7 boys; mean age, 12.54 years) with unilateral or bilateral posterior crossbites involving 3 or more posterior teeth. There was no control group in this study because of the short observation period. The EMG activity was recorded simultaneously from both muscles during swallowing a sip of water and unilateral gum chewing. Results: The mean EMG ( mEMG) activities of the right and left anterior temporal muscles showed no statistically significant difference during swallowing and unilateral chewing. The mEMG activities of the right and left masseter muscles showed a statistically significant difference at unilateral chewing ( P < 0.01). There were a decrease of the mEMG activities for both muscles after RME and an increase during the observation period for unilateral chewing ( P < 0.05). Swallow-related mEMG activities of both muscles showed increases after RME and during the observation period ( P < 0.05). Conclusions: RME affected the EMG activities of the anterior temporal and superficial masseter muscles during swallowing and unilateral chewing. These findings suggest that the alterations in the EMG activities of these muscles should be considered in both the treatment and the the stability of maxillary transverse deficiencies.