Fatigue crack propagation rates were measured in specimens of 4340 low alloy steel (yield strength 225,000 psi) as a function of (1) the water vapor content of the fatigue environment and (2) the amount of fatigue load fluctuation. The fracture surfaces were analyzed by optical and electron microscopy, and attempts were made to correlate the observed fracture topography with the measured rates of crack propagation. Both intergranular and transgranular fractures were observed in the regions of discontinuous crack growth. In several specimens the transgranular areas contained clearly defined parallel fatigue striations. Prior austenite grain boundary fractures were found in flame shaped areas at the crack origins. The combination of a high water vapor content and a low fatigue load fluctuation favored intergranular cracking while a low water vapor content and/or a high load fluctuation resulted in striation type fractures.