An extension shaft is connected to the governor end of a high-pressure steam turbine. This shaft, which drives the primary oil pump impeller in a tubine-generator unit, fractured while in service. Metallurgical analysis included extensive fractographic study using optical, scanning, and transmission electron microscopes. Fatigue striations and beach marks were observed on the fracture surface. Striation spacing measurements were made on carbon replicas taken at various crack depths. The approximate magnitude of the cyclic stresses causing these striations was calculated from data available from scale-model test specimens. Peeling-type cracks initiating from the bottom of the keyway indicative of torsional oscillations were also found. The fracture initiated at the keyway fillet and propagated in high-cycle fatigue mode under combined torsional and bending loads.