Measurements obtained with a profiling laser--Doppler velocimeter reveal the vertical structure of velocity in a wave boundary layer over a sand beach during two periods of approximately stationary forcing, each with a duration of several hours. The measurements indicate clearly the reduction in variance and the increase in phase, relative to the overlying flow, that are expected in an oscillatory boundary layer. The measurements also indicate a distortion of the mean alongshore velocity within the wave boundary layer, which has previously been predicted theoretically and observed in laboratory experiments. The thickness and structure of the boundary layer are approximately consistent with computations based on an existing eddy viscosity model, and model--based best fit estimates of bottom roughness are approximately consistent with existing semiempirical descriptions of wave--formed sand ripples. ¿ American Geophysical Union 1995.