The paper describes a three-dimensional boundary-layer experiment on a rough surface. The data obtained are compared to a reference experiment having the same geometry and flow conditions but with a smooth surface. The boundary layer date are compared using scaling criteria developed from two-dimensional flows. It is found that the results, in terms of scaled variables, agree well in regions where pressure gradients are the dominant cause of three-dimensionality. In regions where shear stresses are more important, significant deviations in profile characteristics appear. A possible interpretation is that the degree of anisotropy is greater in three-dimensional rough boundary-layer flows than in the corresponding smooth-wall case. (Author).