Dislocation-point defect interactions have been investigated in high purity LiF single crystals containing less than 5 ppm of Mg2+, utilising dynamic strain ageing techniques. Two mechanisms of strain ageing have been found. Below 200 °C, the mechanism is one of weak pinning of dislocations by tetragonal strain fields around impurity-vacancy dipoles giving rise to a slight flow stress increase after ageing under stress. Above 200 °C, the strain ageing is due to the strong pinning of dislocations by ordered atmospheres containing impurity-vacancy dipoles and other point defects. This results in the presence of serrated yielding, and a large flow stress increase after ageing under stress. These effects may also be important in previously reported work hardening data in LiF single crystals.