Measurements of the flow field downstream of an upstanding step change in surface roughness are presented. The roughness has the form of two-dimensional square section ribs placed transversely across the floor of the wind tunnel with the first element upstanding from the surface. The surface upstream of the roughness is smooth and is of sufficient length to allow a fully developed smooth wall turbulent boundary layer to be established. The roughness height is approximately 6 percent of the boundary layer thickness on the smooth wall just upstream of the first roughness element. It is observed that downstream of the start of the roughness, the mean velocity profiles inside the internal layer (i.e., that part of the boundary layer which has been affected by the new inner boundary condition) exhibit a linear trend when plotted in the form U versus y1/2 . Remarkably, it is also found that a linear trend is exhibited by points in the “undisturbed” boundary layer outside the internal layer when plotted in the above manner, and that the slope in the undisturbed layer differs from that in the internal layer. The undisturbed layer slope appears to depend on conditions upstream of the roughness. It is suggested that the point of inter section of the two straight lines (the “knee” point) on the U versus y1/2 plot may be used to define the edge of the internal layer. Turbulence intensity distributions and spectra are presented from which it is deduced that the internal and external layer structures are largely independent and that stream-wise length scales in the internal layer over the rough wall are reduced significantly below those at the equivalent station over a smooth wall.