Cross sectional microhardness maps of cast-to-shape tensile specimens of rectangular and circular cross-sections have been obtained for a Mg-9 mass% Al alloy. The hardness is generally higher near the surface and at the corners of the cross-section in comparison with that at the centre. The lower hardness values at the casting core are accounted for by the coarser solidification microstructure and the concentration of porosity. The cross sectional mapping shows that the harder surface layer is generally uneven, questioning the concept of a well defined, uniform and continuous skin. Physical reasons for these features of the casting’s skin are discussed in terms of the grain microstructure.