Two cast Mg–Al–Ba–Ca alloys, DieMag633 and DieMag844, are the focus of this study. Their microstructureMicrostructure consists of a eutectic Ca-rich structure, the lamellar Al₂Ca, and of a hard, compact and brittle interdendritic Ba-rich phase, the Mg₂₁Al₃Ba₂ phase. Both phases have a much higher hardness than the α-Mg matrix and strongly influence the crack propagationCrack propagation under quasi-static and fatigueFatigue loading. Mostly interdendritic crack propagationCrack propagation has been found—the crack growth follows the coherent interdentritic network of second phases. Transgranular cracking within the compact Ba-rich phase is very pronounced, showing many microcracks within this phase. The phases strain harden in a different amount. Transdendritic cracks are found only under cyclic fatigueFatigue loading, and the cracks transit from the second phases into the α-Mg dendritesDendrites. The lower crack growth rate and the stress increase at the interface to the dendritesDendrites seem responsible for the transdentritc crack propagationCrack propagation. The influence of the chemical composition of the alloying elements and the fatigueFatigue stress ratios are discussed.