The locomotor and postural behavior of Hylobates lar was studied in a seminaturalistic environment at the New York Zoological Park. A particular locomotor pattern, one-armed brachiation, was observed and filmed. Analysis indicated that the occurrence of one-armed brachiation as a preferred locomotor pattern was rare and was limited to the carrying of food. The limb and joint movements of one-armed brachiation closely resembled those of two-armed brachiation with differences occurring in the angular rotations of the support and the free arms. Analysis showed how a gibbon could maximize its forward momentum during one-armed brachiation. The adaptive value of one-armed brachiation is discussed in reference to brachiating while carrying food and to brachiating with a fracture of a forelimb. Finally, one-armed brachiation is discussed as an example of the concepts of locomotor totipotentiality and locomotor habit.