In many geophysical flows the passage of a turbulent fluid over a wavy surface significantly affects quantities of interest, such as momentum flux, mixing, and transport of scalars. Large-eddy simulation is used here to investigate neutrally stratified flow over a wavy bed. A wavy surface tends to organize the turbulent flow by imposing its wavelength and phase on the near-wall field. Both instantaneous turbulent structures and turbulence statistics provide evidence of this organization. For example, a vortex identification method applied to the instantaneous flow field reveals streamwise vortices that are locked into a phase relationship with the wavy boundary. These vortices may be linked to a G¿rtler instability mechanism caused by the wavy wall. Statistical analyses are also presented to gain insights into the effect of the wavy surface. In particular, we compare the effects of three different hill heights. Finally, a brief analysis of the components of drag is given. ¿ 2001 American Geophysical Union