Oxygen uptake ( O₂) at steady state, heart rate and perceived exertion were determined on nine subjects (six men and three women) while walking (3–7 km · h⁻¹) or running (7–14 km · h⁻¹) on sand or on a firm surface. The women performed the walking tests only. The energy cost of locomotion per unit of distance (C) was then calculated from the ratio of O₂ to speed and expressed in J · kg⁻¹ · m⁻¹ assuming an energy equivalent of 20.9 J · ml O₂ ⁻¹. At the highest speedsC was adjusted for the measured lactate contribution (which ranged from approximately 2% to approximately 11% of the total). It was found that, when walking on sand,C increased linearly with speed from 3.1 J · kg⁻¹ · m⁻¹ at 3 km · h⁻¹ to 5.5 J · kg⁻¹ · m⁻¹ at 7 km · h⁻¹, whereas on a firm surfaceC attained a minimum of 2.3 J · kg⁻¹ · m⁻¹ at 4.5 km · h⁻¹ being greater at lower or higher speeds. On average, when walking at speeds greater than 3 km · h⁻¹,C was about 1.8 times greater on sand than on compact terrain. When running on sandC was approximately independent of the speed, amounting to 5.3 J · kg⁻¹ · m⁻¹, i.e. about 1.2 times greater than on compact terrain. These findings could be attributed to a reduced recovery of potential and kinetic energy at each stride when walking on sand (approximately 45% to be compared to approximately 65% on a firm surface) and to a reduced recovery of elastic energy when running on sand.