The mechanical behaviour of Mg-5.1 wt % Zn alloy single crystals was studied in the 4.2 to 300° K temperature range. Quenched crystals have activation energies and volumes best associated with the cutting of small clusters of Zn atoms by dislocations. Fully hardened crystals contain fine β 1′ and occasionalβ 2′ precipitates with an averageβ 1′ interparticle spacing of 330 to 660 A. Strengthening in these crystals is mainly ascribed to the cutting of β 1′ particles by dislocations. In the overaged condition β 1′, β 2′ and equilibrium β particles are present and lead to a considerable temperature-dependence unusual for an overaged condition. Analysis of this temperature-dependence suggests that below 77° K the relatively easy cutting of β 1′ particles by dislocations takes place in addition to the cutting of β 2′ and β particles. Above 77° K the difficult cutting of β 2′ and β particles alone controls the deformation, β 1′ being more easily cut with the aid of thermal fluctuations.