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Fatigue crack propagation behaviour of compact tension specimens of DSX40M alloy has been investigated in air at room temperature and at 850degreesC. The results indicate that the crystallographic cracking occurs on two or more planes simultaneously at room temperature. Slip bands are observed along the crack. Crack growth is affected by the temperature. The fatigue crack growth is higher at elevated temperature than room temperature. Microcracks start at the precipitates/eutectic and join the fatal crack. The number of planes intersecting each other determines the different microscopic features on the cleavage fracture surface. Dislocation structure in the plastic zone underneath the crack propagation at room temperature and at 850degreesC has been investigated by transmission electron microscopy. Dislocation pile up starts at lower DeltaK and dislocations are moving in a planar slip and few of the dislocations form network in the final broken area at room temperature. At 850degreesC the dislocations are piling up at lower DeltaK and dislocations are dispersing and forming network at higher DeltaK. The complete network formation was observed in the final broken area at 850degreesC. The dislocation density is very much higher at 850degreesC compared to that at room temperature. (C) 2002 Kluwer Academic Publishers.