The dynamics of a three-dimensional axisymmetric blu -body wake are examined at low Reynolds regimes where transitions take place through spatio-temporal symmetry breaking. A linear stability analysis is employed to identify the critical Reynolds num- ber associated with symmetry breaking, and the associated eigenmodes, known as global modes. The analysis shows that the axisymmetric stable base ow breaks the rotational symmetry through a pitchfork m = 1 bifurcation, in agreement with previously reported results for axisymmetric wakes. Above this threshold, the stable base ow is steady and three-dimensional with planar symmetry. A three-dimensional global stability analysis around the steady re ectionally symmetric base ow, assuming no homogeneous direc- tions, predicts accurately the Hopf bifurcation threshold, which leads to asymmetric vortex shedding. DNS simulations validate the stability results and characterize the ow topology during the early chaotic regime.