Introduction: Habits are learned patterns of muscle contraction by nature very complex. Certain habits, stimulate growth of the jaws, for example, the normal action of the lips, normal swallowing, phonation and mastication.Oral habits such as finger sucking, the bricomania, onicofagia, the habit of biting the tongue or purported to alter the normal development. Objective: To identify pernicious oral habits that are common in the Mexican child population and the type of malocclusion with which associates. Materials and Methods: Descriptive study in a population of 148 preschool children, 49% females and 51% male, mean age was 5.0. Results: Pernicious oral habits more often in this population was preschool onicofagia second digital sucking. With regard to the type of bite, it was observed that 9.4% had abnormal anterior open bite, bilateral crossbite, and posterior unilateral crossbite. The age of 5 years has the highest number of abnormalities of the type of bite presents. Of all preschoolers studied 35% presented oral habits perniciosos.Un issue to draw attention is that 10.3% of preschoolers who were not detect a habit. Conclusions. The habit of gnawing the nails, is usually a replacement of the act of sucking the finger or pacifier. This linked with serious emotional disturbances and not cause problems of occlusion, and pressure that occurs only in the longitudinal axis of teeth, the predominance of vertical growth of the face, which results in conjunction with atypical deglutition is the emergence of the anterior open bite.