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The mechanical properties of LiF single crystals with relatively low impurity content are compared with those containing ∼75 ppm of magnesium. These crystals represent the extremes of material that have been obtained from the Harshaw Chemical Company during the past six years. The recently produced, ``pure'' crystals are very soft (80–100 g/mm2 critical resolved shear stress), and insensitive to heat treatments; and their flow stress varies slowly with test temperature. The impure crystals are much harder as received (∼1500 g/mm2) and respond to heat treatment in a manner that suggests precipitation hardening. The temperature dependence of the flow stress is much different in slowly cooled and rapidly cooled impure crystals. Whereas the slowly cooled crystals are much harder at room temperature, air-cooled crystals in which the magnesium is present in vacancy-impurity pairs are harder at low temperatures. Two questions that must be answered are: (1) How does such a small amount of impurity present in aggregates produce a large hardening at room temperature? and (2), how do isolated vacancy-impurity pairs produce the large, temperature-sensitive hardening at low temperatures? The effects of impurities must be understood in terms of how they resist the motion of fresh dislocations. One possibility that should be further explored is that impurities in these crystals affect the resistance to dislocation motion by altering the steady-state number of jogs on the screw dislocations. The existence of many jogs and the fact that they contribute to the flow stress have been previously established.