A study of the effect of precipitation hardening on the yield behaviour in torsion of an AlZnMg alloy has been carried out. Two modes of unstable yield behaviour were observed. As-quenched and well over-aged specimens exhibited serrated yielding. Specimens aged at elevated temperatures exhibited a single yield instability and the extent of the flow localisation was found to be dependent on ageing treatment. In specimens aged through peak strength conditions the flow localisation resulted directly in failure. In over-aged specimens the flow localisation stabilised into a deformation band which propagated at least part way along the gauge length prior to failure. Extensive over ageing resulted in the re-occurrence of serrated yielding. Abrupt shear failures with little preceding localised flow were observed in naturally aged specimens.