Solution strengthening in single crystals of magnesium containing Zn in amounts up to 0.45 at. % has been investigated over the temperatures between 78°K and 423°K. The critical resolved shear stress for basal slip was found to increase with the square root of the solute concentration in two linear stages having a transition at 0.025 at. % Zn. No solution hardening was observed above 330°K when the solute content was less than 0.025 at. % Zn. Using transmission electron microscopy of thin foils the basal dislocation density has been found to increase linearly with the square root of the solute concentration. This increase however, could not account for the observed variation in the athermal component of the CRSS with alloying. The increase in the long range stress field can be best explained in the light of Fleischers friction stress mechanism. The rate theory has been used to identify the short range obstacles, and the low temperature solution hardening is discussed.