The creep behaviour of MC2 single crystal superalloy has been studied at 1150°C/80 MPa, with an applied load along  axis. The resulting dislocation microstructures were examined by transmission electron microscopy. The occurrence of a type dislocations (with a zero Schmid factor) within the ordered γ' precipitates is often observed. It is shown that those dislocations moved by a climb process, based on a mechanism involving two dislocation systems and vacancy exchanges, as proposed in the literature. We calculate the vacancy fluxes associated with such a mechanism and show that the vacancy transportation can be easily insured by a simple diffusion process. This calculation shows that the diffusion and climbing steps do not seem to be the creep rate controlling mechanisms for those situations in MC2 alloy.