Measurements are described of the fluid velocities close to a rough bed oscillating in its own plane in a chamber with superimposed mean drift. The measurements were made in air with a hot-wire anemometer. It was found that over a wide range of conditions the common assumption that the combined flow may be treated as the vector sum of the steady and oscillatory flows separately is incorrect. In all cases investigated, the drift near the bed was less than that which would have been expected in the absence of non-linear interaction between the oscillatory and steady flows. For certain conditions the drift near the bed was actually in the opposite direction to that of the superimposed flow.