This study describes the patterns of activity of hip abductor and adductor muscles, and relates their activity to the frontal plane motions of the hindlimbs during unrestrained walking in the cat, to provide insight into the function of these muscles in maintaining stability during walking. Electromyographic activity was recorded from hindlimb muscles while cats walked across a walkway. Four video cameras were used to record the movement of the animal in three dimensions. To further delineate the role of the hip abductors and adductors in regulating frontal plane movements of the legs, medial-lateral translations of the walking surface were periodically introduced. During walking, the hip abducts throughout much of the stance phase and adducts during swing. Normally, the abductors and adductors are co-active during much of the stance phase and are quiescent during swing. Consequently, the adduction observed during swing is likely the result of passive events. It is argued that the activity of the hip abductors during stance phase plays a prominent role in regulating frontal plane motion of the legs during walking. However, when medial-lateral stability is disturbed both the hip abductors and adductors respond to stabilize the frontal plane motion of the body mass while also adjusting the frontal plane swing trajectory and subsequent paw placement. The balance corrective reactions observed in the cat following medial-lateral perturbations of the support surface reasonably approximate the reactions observed previously in humans, indicating that the cat is a reasonable model to explore the neural mechanisms of lateral stability during walking.