Lithium fluoride crystals were exposed to cobalt‐60 gamma radiation at 300° and 78°K. Changes of the flow stress and of the optical absorption were measured in an effort to relate the hardening to identifiable color centers.In crystals irradiated at 300°K there was found to be a good correlation between the flow stress and the concentration ofFcenters. The magnitude of the hardening, however, suggested that it was caused by interstitial defects. Comparison of the hardening produced by additive coloring with that produced by irradiation indicated that the hardening defects were interstitial fluorines. This direct relation between interstitials andFcenters supports a Varley type of mechanism forF‐center production in the bulk crystal. Some limited experiments with NaCl and KCl showed qualitatively the same behavior and suggest that the sameF‐center production mechanism may occur in them.Irradiation of LiF at 78°K produced stable trapped‐hole centers and pulse annealing experiments showed that theVkcenter (self‐trapped hole) contributed to the hardness at low temperatures. A sharp decrease of flow stress was observed at about 110°K where the paramagnetic resonance experiments of Kanzig and the optical measurements of Wiegand and Smoluchowski indicate disappearance of theVkcenter.