The formation of regular surface waves in injection moulded polystyrene was investigated as a function of the moulding conditions and the molecular weight distribution of the polymer. The injection velocity and the position of measurement relative to the gate of the mould have the greatest effect on the average roughness measured with a Talysurf machine. The packing pressure has only a minor effect in reducing the roughness. It is proposed that the waves occur if there is time for surface buckling in the fountain flow at the melt front. The waves are shown to have negligible effect on the unnotched impact strength, which is dominated by molecular orientation, so the waves merely detract from the surface appearance of the moulding.