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Castings were made in an unfluxed commercial purity Mg alloy in plate molds with both welldesigned and poorly designed running system features. The poor running system design was associated with more entrainment of the surface oxide film during mold filling and produced a greater spread in ultimate tensile strength (UTS) and pct elongation values than the castings produced with the good running system design. This is also an effect found in Al alloys where it has also been associated with entrainment of the surface oxide film during mold filling. Scanning electron microscopy of the fracture surfaces of tensile test bars cut from the cast plates revealed oxide film defects comprised of folded over and symmetrical MgO films, similar in form and structure to the double oxide film defects found in Al alloys.