To improve our understanding of the rheology of solid semi-crystalline polymers, descriptions of the deformation-induced microstructural reorganization mechanisms with precise and local quantitative data are needed. The novel results presented in this paper for high-density polyethylene (HDPE) were obtained in situ on a coherent synchrotron beamline specifically developed to allow very fast scanning of a specimen under tensile test. From the recorded small angle X-ray scattering (SAXS) patterns, a quantitative index characterizing the microstructure local anisotropy was calculated. With the scanning operating mode, many different material points could be studied. These material points were subjected to various deformation paths in the plastic regime due to the necking development and its propagation. Their positions and strain evolutions were obtained through digital image correlation (DIC). With an appropriate analysis coupling the bulk-averaged SAXS and DIC surface measurements, the microstructural anisotropy index is shown to have a given value at a given true strain. This means that the microstructure morphology is only governed by the current strain level. (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 170-181