STRUCTURES possessing spatial asymmetry should act as pumps in the presence of dissipation alone(1-4), without the need for macroscopic forces or temperature differences(5) to drive vectorial motion. It has been shown theoretically(2-4,6,7) that particles subjected to an asymmetric periodic potential can display net directional motion even if the space-averaged forte is zero. Here we demonstrate such behaviour experimentally. We have studied the behaviour of colloidal particles suspended in solution and exposed to a sawtooth dielectric potential which is turned on and off periodically. The particles exhibit net motion with a velocity that depends on their size, suggesting applications in separation processes for objects in the size range 0.1-5 mu m-a range that includes biological structures such as viruses, cells and chromosomes(8). We furthermore point out the analogy between our device and motor protein assemblies.