The long-term properties of PalphaSAN/PMMA blends have been examined by conducting fatigue tests on a fracture mechanics-type specimen. Initial testing on the individual constituents showed that the crack initiation threshold was higher in the PMMA polymer than in the PalphaSAN system. Alloying these polymers resulted in the crack initiation thresholds between those of the two base polymers. A scanning electron microscopy study of the fracture surfaces indicated that fatigue loading resulted in the formation of well defined striations, the spacing of which depended on the crack velocity and morphology. The fatigue striations in a fine-structured blend remained straight and did not appear to be modified by the presence of the secondary phase. Crack extension in a coarser structure depended upon the characteristics of the particular phase in which the crack was extending. It was observed that the striations in the less ductile PalphaSAN phase were more closely spaced as well as being curved. It is believed that the crack propagated more rapidly in this phase resulting in a split rate crack velocity within the polymer blend.