The structure of the membranous wall of the trachea was studied in 560 tracheas from children aged 24 weeks of gestation to 10 years. The transverse muscle fibres are uniformly arranged. This is in marked contrast to the longitudinal smooth muscle which varies throughout the entire tracheal length. In 16% of tracheas, the longitudinal muscle was absent, while in the other 84% its distribution was variable with the muscle being present in the lower half of the trachea in approximately 70% of cases. It is suggested that the longitudinal muscle may be active in preserving the stability of the tracheal wall, preventing tracheal collapse and obliteration of the lumen, particularly in the intrathoracic part of the trachea.