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Single peak overload retardation of corrosion fatigue crack growth under constant Delta K control was studied for A537 steel in air and in a 3.5% NaCl solution at the applied cathodic potential of 1400 mV(SCE) and in free corrosion condition. The crack closure behaviour was examined by the speckle interferometry technique at different stages of fatigue crack propagation. Scanning electron microscopy was used to observe the morphologies of fracture surface. The experimental results showed that, in all conditions, overload retardation occurred and the retarded propagation distances of crack were longer than the plastic zone size caused by overload at the crack tip. In air or in 3.5% NaCl at the cathodic potential, delayed retardation occurred after applying a peak overload, the crack growth consisted of three stages: crack extension due to overload; decelerating in crack growth; and resumption of crack growth rate. Immediate retardation occurred when specimens were tested in a 3.5% NaCl solution in free corrosion conditions. Crack extension due to overload was not observed. Under both cathodic polarization and free corrosion condition, the overload retardation phenomena were not as obvious as that in air. The retarded propagation distance and the retardation degree were both smaller than those in air. The mechanisms for overload retardation of corrosion fatigue crack growth under the control of hydrogen embrittlement and anodic dissolution were discussed on the basis of analyses of crack closure, residual compressive stress and crack tip blunting. (C) 1998 Elsevier Science Ltd.