The effects of neodymium‐dopant additions on the behavior of dislocations in calcium fluoride were studied. Velocity measurements were carried out on edge and screw dislocations as a function of stress, temperature, and dopant content. It was observed that screw dislocations can be the more mobile type in CaF2, depending on the stress and concentration of aliovalent impurities present. The mobility of screw dislocations relative to edges was reduced significantly by 70 ppm Nd3+. An apparent transition in the drag mechanism from extrinsic to intrinsic was observed for edge dislocations.