Cineradiographic analysis of the posture and movements of the femur and pelvis in representatives of three carnivore families (Procyonidae, Felidae, Canidae) reveals discrete locomotor patterns during walking. Raccoons (Procyon tolor) abduct the femur substantially, and engender large pelvic movements. Canids (Vulpes fulva, Canis familiaris) abduct the femur slightly, whereas cats (Felis domestica) employ only a sagittal excursion of the femur with the least pelvic oscillation. Casted replicas of the femora and pelves of these species, fixed in the various characteristic postures and then sectioned, demonstrate differences in articular relationships. The depth of the acetabulum, location of the fovea capitis femoris, and configuration of the articular surface on the femoral head vary by species according to the femoral posture and excursions employed in locomotion and other activities. Three patterns of locomotor adaptation are identified on the basis of these data and behavioural observations. Ambulatory carnivores (e.g. raccoons) are adapted for a wide range of femoral postures, and particularly for abduction used in climbing. In contrast, cursorial carnivores are specialized for a narrower range of hindlimb movements. The canine pattern is the most restricted; the hip structure accommodates little more than the modest degree of abduction employed in terrestrial locomotion. The feline pattern, although characterized by the most adducted limb posture during walking, nonetheless retains structural adaptations for a wider range of femoral postures than in canines which may be employed in climbing and in predatory activities. The habits of certain fossil carnivores are interpreted on the basis of these patterns. Creodonts and a Paleocene miacid (Carnivora) show distinctly ambulatory adaptations, whereas later miacids developed various cursorial characters. The early Oligocene canid Hesperocyon gregarius had a hip structure with features comparable to those in modern felids and canids.