The fatigue crack propagation (FCP) behaviour of polystyrene (PS) and 95/5 PS/high density polyethylene (HDPE) was studied at cycling frequencies (f) of 0.2, 2 and 20 Hz. At the latter two frequencies, the fatigue crack growth rates (FCGRs) in the blends were lower than in PS, especially when a styrene/ethylene-butylene/styrene (SEBS) triblock copolymer was added as a compatibilizer. The fractographic features observed were analysed in detail.Discontinuous growth bands (DGBs), associated with the fracture of crazes, formed at low FCGRs in PS and at low and high FCGRs in the blends. Large dimple-like features formed at intermediate FCGRs and fatigue striations at high FCGRs were observed in PS. The observations indicated that the reduction in FCGR when HDPE was added to PS was associated with the presence of stretched HDPE particles which fractured behind the crack front, with increased particle-matrix adhesion favoured by the compatibilizer. An increase in cycling frequency decreased the FCGRs, with the fractographic observations indicating that this effect was associated with a decrease in the time-dependent deformation in the fracture process zone slightly ahead of the crack tip.