The properties of a riblet surface at a Mach number of 1.25 are described. Direct measurement has been made of the surface shear stress on a riblet surface attached to a skin-friction balance mounted beneath the turbulent boundary layer growing along a wall of the RAE 2 ft×1 1/2 ft variable density transonic wind tunnel. Comparison of the data with that for a smooth wall indicates a reduction of about 7% in skin friction at optimum flow conditions. Misalignment of the riblet surface with the flow direction reduces this benefit and beyond 30° misalignment a drag penalty occurs. Analysis of the velocity profiles indicates that compressibility effects may be accounted for with the use of an appropriate transformation.