Cross-sectional microhardness maps of cast-to-shape flat tensile specimens have been obtained for a binary Mg-3.44 mass% La alloy. Higher microhardness numbers were generally found near the casting surface, at the corners and along the segregation band. The higher hardness values were ascribed to the finer solidification microstructure near the surface and to localized positive macro segregation. The majority of lower hardness numbers was found at the core region. Lower hardness values were ascribed to the coarser grain size prevalent at the core and to dispersed microporosity. The non uniformity of the harder surface layer in both depth and hardness appeared related to local homogeneities in the grain size distribution caused by the scattered presence of large externally solidified grains.