The energy expenditure and gait characteristics of eleven subjects who had a unilateral arthrodesis of the hip and ten subjects who had a unilateral arthrodesis of the ankle were measured during a twenty-minute walking trial. Oxygen consumption was determined and heart rate, respiratory rate, and step frequency were monitored. The average walking speed for both groups was sixty-seven meters per minute, which is 84 per cent of normal gait velocity. After an arthrodesis of the ankle, the average rate of oxygen consumption was 12.0 milliliters per kilogram of body weight per meter, which is 3 per cent more than normal. The oxygen cost averaged 0.166 milliliter per kilogram per meter, resulting in a gait efficiency of 90 per cent. After arthrodesis of the hip, the mean rate of oxygen consumption was 14.9 milliliters per kilogram of body weight per meter, which is 32 per cent greater than normal. The oxygen cost, 0.223 milliliter per kilogram per meter, represented a gait efficiency of 53 per cent. The physiological energy expenditure by the subject after an arthrodesis of the hip was found to be significantly greater than after an arthrodesis of the ankle and was more than that reported previously after a total hip arthroplasty for unilateral degenerative disease.