The deformation and damage mechanisms arising during thermal-mechanical fatigue (TMF) of the single-crystal superalloy CMSX-4 have been investigated, both in the virgin condition and after long-term exposure at 1000 °C. Fundamental differences in the mechanical response due to ageing have been discovered, which are attributed to the tendency for the precipitation of topologically close-packed phases during deformation. In the virgin condition, the deformation during TMF is very localized and concentrated to twin bands which extend over the complete cross-section of the specimen; at the interception of these bands, the material is prone to recrystallization. The aged material on the other hand shows a much more dispersed deformation behaviour in which the length and thickness of the twins are much smaller and no recrystallization is found. Instead, significant local misorientations-implying crystal rotation-are observed in the aged material after rupture. © 2009 Acta Materialia Inc.