The effect of microstructure on the high temperature low cycle fatigue deformation mechanisms of an advanced Ni-based disk superalloy was studied using TEM characterization methods. In order to track the evolution of these mechanisms, specimens were interrupted after a limited number of cycles and were not run to failure. Both fine and coarse precipitate microstructures were examined, corresponding to a fast or slow cool, respectively, from the gamma prime solvus temperature. The operative deformation mechanisms are correlated with the precipitate structure, number of cycles and testing temperature.  The observed mechanisms include stacking faults, dislocation bands, and microtwins.  A rationale for the observed mechanisms in terms of time-dependent damage processes will also be discussed.