OBJECTIVE The aim of this study was to evaluate the electromyographic activity of both the temporalis and masseter muscles and the mastication type of patients with skeletal unilateral posterior crossbite before and after orthodontic treatment and speech therapy. METHODS A total of 14 patients with skeletal unilateral posterior crossbite (eight females and six males), between 6-13 years of age, underwent electromyographic evaluation of their masseter and temporalis muscles in mandibular rest, habitual mastication and isometry. The subjects were assessed with regard to mastication type before and after orthodontic treatment and speech therapy. The data obtained during mandibular rest and habitual mastication were normalized in terms of the mean values of isometry. The Student's t-test was used for paired samples to compare the mean values of electromyographic activity (p < 0.05). RESULTS The masseters during habitual mastication presented higher electromyographic activity after both treatments (p = 0.0458). There was no significant difference between the contralateral masseters in terms of mandibular rest or habitual mastication before or after either treatment (p > 0.05). During habitual mastication, after the treatments, the temporalis muscle on the malocclusion side showed higher electromyographic activity than the contralateral side (p = 0.0263). Prior to therapy, all of the patients exhibited chronic unilateral mastication (n = 14) and 13 patients exhibited bilateral mastication after treatment. CONCLUSIONS Orthodontic intervention combined with myofunctional therapy in patients with skeletal unilateral posterior crossbite provided an increase in the electromyographic activity of the masseter and temporalis muscles during mandibular rest and habitual mastication, with predominantly bilateral mastication.