Many anthropologists and anatomists have claimed that the human gluteus maximus is a functionally and structurally unique muscle, but there is not agreement on the actual characteristics of the muscle which do distinguish man from other primates. In this paper the superficial gluteus in a wide range of primates is discussed and those traits entirely unique to man are identified. The morphological specializations of the human gluteus maximus are confined to its cranial portion. This part is thicker in man than in any other primate; it has a new and firm origin, a modified ascending tendon of insertion, and an additional new insertion into the overlying fascia lata. Such changes improve the ability of the gluteus maximus to participate in controlling lateral stability of the trunk, and it suggested that this is the function which has been selected for in human evolution.