Stokes calculated the force exerted by the surrounding fluid on a sphere and on a cylinder in oscillating motion. Although these results are valid only if the Reynolds number Re is very small, Re 1, all the tests on macroscopic spheres have been made with Re larger than 20. Here, we describe an experiment which measures the drag force on an oscillating sphere with small values of the Reynolds number, down to Re ≈ 0.03 for the smallest sphere studied here while the Stokes' number St is large, between 150 and 1500. Our measurements are in very good agreement with Stokes' result and, in particular, they exhibit the quadratic dependence of the force with the sphere radius when this radius is larger than the viscous penetration depth δ.