Abstract Introduction: The literature emphasizes the importance of acquiring good motor coordination in the early years of life and its relationship with physical fitness and physical activity during adolescence and adulthood. Objective: To analyze the effect of biological maturation on the motor coordination in boys. Method: The sample was composed by 203 boys between 11 and 14 years old. Height, body mass, sitting height, waist circumference (WC) and skinfolds were measured. Somatic maturation (SM) was assessed by maturity offset (estimated age at peak height velocity). The gross motor coordination was evaluated by Körperkoordinationstest für Kinder (KTK) battery. Results: The SM exerted an effect on the walking backward on balance beams (WB) mediated by the WC. Conclusion: The results showed that the performance of boys in the WB was negatively influenced by the greater volume of fat in the trunk related to a more advanced state of SM.