The mechanical behaviour of an Al4.5wt%Cu alloy in the mushy state has been studied using an axisymmetric shearing experiment. The technique is based on the torsion of a hollow cylinder with a reduced section at mid-height. This thin section, where the deformation and fracture is controlled, is reheated by induction to the desired temperature at rates sufficiently high to minimise the influence of coarsening on the microstructures. Low deformation rates can be investigated using this shear test, due to the fact that the liquid is not squeezed out in the thin section. The coherency limit and the overall mechanical behaviour of a given alloy can be studied as a function of the microstructure. In the case of an equiaxed sample, deformation is mainly concentrated in the intergranular regions of the thin section. Mechanisms of deformation and fracture are strongly affected by the volume and distribution of the liquid phase. The mechanical behaviour is strongly influenced by the amount of necking among the grains. For columnar microstructures sheared in a plane perpendicular to the dendrites trunks, deformation is homogeneously supported by the solid trunks leading to higher shear resistance. Columnar microstructures sheared in a plane parallel to the dendrites trunks exhibit a resistance in between the two other cases.