The effect of lithium addition on the flow stresses for basal and nonbasal slip in magnesium crystals has been investigated by tensile tests at various elevated temperatures, using Mg-Li alloy single crystals of h.c.p. structure. The critical resolved shear stress for basal slip is found to increase almost linearly with lithium concentration, while that stress for nonbasal slip is found presumably to decrease by alloying with lithium at temperatures below about 200°C. The solid solution hardening observed in basal slip system can be interpreted most reasonably by Suzuki’s chemical interaction mechanism among several current theories on solid solution hardening. The abnormal behavior observed in nonbasal slip may be qualitatively interpreted by the cross glide mechanism. In order to explain quantitavely the obtained results as to nonbasal slip, however, the Friedel model concerning cross glide mechanism is necessary to be modified.