The conventional long wavelength instability analysis is modified by the addition of a suitable triaxiality term so that it continues to apply in the presence of transverse stresses. The classical Lagrangian expressions describing the kinetics of flow localization are unsuitable for this type of analysis and the appropriate Euler formulations are derived. Experiments involving neck propagation during the tensile testing of high density polyethylene rods are described. The results are in good agreement with the predictions of the theory, confirming the importance of the triaxiality corrections. The discrepancies that remain are attributed to: 1. (i) the extent to which the flow behaviour of HDPE is not consistent with the ‘engineering’ flow laws on which the instability analysis is based; 2. (ii) the inadequacy of the classsical Bridgman analysis at very large strains.