A series of mechanical tests was used to characterise the evolution of yield strengths, r-values and work hardening during the natural ageing of a commercial 7075 aluminium alloy. The results show that there is a significant underlying change in anisotropy between the original O-condition and solution heat treated tempers. In the absence of recrystallisation, the abrupt change in anisotropy appears to have been caused by the modification of the precipitate during the solution heat treatment. The degree of induced anisotropy remained constant throughout the natural ageing process from 15 to 120 min. A series of phenomenological functions is presented for the purpose of interpolation between the original test data points. These empirical functions are shown to predict the yield behaviour of the naturally aged material within the observed experimental scatter.