Introduction: Mouth breathing seems to increase the cervical muscles activity, due to postural adaptations necessary to reduce the airways resistance. Objective: To evaluate the electrical activity of sternocleidomastoid and trapezius muscles in mouth breathing children and to compare them with nasal breathers, in rest and postural correction. Material and Methods: For this, an electromyographic study of these muscles was accomplished in 46 children, ranging from 08 to 12 years old, of both sexes, and divided in two groups: Group I, with 26 mouth breathers and, Group II, with 20 nasal breathers. Results: There was a significant higher activity of the studied muscles in both situations in Group I when compared to Group II. Conclusions: It is concluded that mouth breathers increase the cervical muscles activity in order to adequate the head and neck position and, this way, reducing the airways narrowing.