The fracture characteristics of Mg-3.7Th-0.4Zr and of Mg-0.6Zr have been investigated. The ternary is a precipitation hardenable system exhibiting a coherent transition precipitate, while the binary forms a stable dispersion of zirconium hydride when heated in an atmosphere of hydrogen. The ternary was investigated in four different heat treated conditions which provided two levels of yield stress. A solution treated alloy and an overaged alloy provided one yield stress, while the other yield stress level was attained by interrupting the heat treatment prior to optimum aging and by slightly overaging. The binary alloy was examined in the hydrided and solution treated conditions only. Specimens were fractured at —196, 23, and 210 C by tension testing. Further information concerning the influence of twins and the initiation of microvoids was acquired by differential temperature tests, where specimens were deformed at one temperature to a predetermined strain followed by deformation to fracture at another temperature. Electron fractography was accomplished on all fractured specimens in order to determine the mode of failure, the influence of precipitate and hydride dispersion on the size and distribution of dimples, and the amount of serpentine glide as a function of temperature and specimen condition. In aged specimens voids observed by light microscopy were up to two orders of magnitude larger than dimples sizes determined by electron fractography. It appeared that dimples are produced by overload failure of material bridging the voids.