Metallographic investigation of the discs used in tests to simulate gear tooth contact has revealed the occurrence of several microstructural features that are present only in fatigued specimens: e. g. dark etching regions ( DER) and white etching bands ( WEB). In addition, a new type of microstructural constituent has been observed close to the surface, below asperities. This feature is referred to as the plastic deformation region ( PDR). For the first time the hardness of these fatigue-induced phases was accurately determined by the nanoindentation technique. It has been found that DER initiates at the prior austenite grain boundaries. Fatigue cracks initiate and propagate preferentially at the boundaries of PDR and ultimately lead to the formation of micropitting. (c) 2005 Springer Science + Business Media, Inc.