AZ61 Mg alloy was hot-forged and subsequently hot-rolled to a thickness of 1 mm. Tensile anisotropy was investigated at room temperature within the plane of the rolled sheets. When the basal planes were tilted by more than 16.5° towards the tensile axis, basal 〈a〉 slip was a dominant deformation mechanism and a small elongation of 8% was obtained. Otherwise, prismatic 〈a〉 slip became a dominant deformation mechanism and a large elongation of more than 20% was obtained. The basal-plane tilt angle was considered as a geometrical criterion for the activation of the prismatic 〈a〉 slip at room temperature. The corresponding Schmid factor ratio, or equivalently the critical resolved shear stress ratio of prismatic to basal slip, was found to be 1.5-2.0.