The effect of particle roughness on turbulence structure in a water boundary layer in an open channel flow is experimentally investigated using a laser-Doppler velocimeter (LDV). As a wall boundary, a fixed-or a free-particle wall is used against various particle-occupation densities epsilon. The results show that for the same epsilon the friction velocity on the fixed-particle wall is larger than that on the free-particle wall, and that both friction velocities have maxima in the region of 0.1 less than or equal to epsilon less than or equal to 0.2. The Reynolds stress and turbulence intensities outside the roughness sublayer, normalized by the friction velocity, are independent of the particle roughness. The u-v quadrant decomposition technique elucidates that ejection events are superior to sweep events in the vicinity of the wall over smooth and free-particle walls, whereas over the fixed-particle wall the sweep events are more dominant in the production of the Reynolds stress than the ejection events. (C) 2001 Elsevier Science Ltd. All rights reserved.