The fatigue crack propagation (FCP) mechanism of an ABS was examined under tension-tension loading as a function of load level. The crack was always preceded by a damage zone consisting of assemblies of stress-whitened lines. Crack growth was predominantly through the root damage line, with occasional jumps into neighbouring lines. These jumps occurred most frequently in the medium load level test. Transmission electron microscopy (TEM) analyses showed the damage within the stress-whitened lines to consist of elongated, and occasionally ruptured, rubbery domains at the specimen surface, with the appearance of extensive crazes bridging their interstices toward the mid-thickness. The craze intensity (size and number) increased with decreasing load level. The crazes manifested themselves on the fracture surface as a patch morphology, which was increasingly pervasive with decreasing load level. In the rougher region that followed, ductile tearing of the matrix is believed to be promoted by rubber cavitation a nd the merging of crazes. Normal fatigue striations indicate crack advance by greater fractions of the damage zone length at higher load levels.