The changes in average birefringence and internal stress during retraction at 85°C have been measured for four different oriented polystyrene monofilaments. The data for the different filaments all appear to fall on a single curve when plotted as internal stressvsbirefringence. Data on internal stress and birefringence obtained for thirteen different oriented filaments (including the four above) in their initial uncontracted state also fall on the same curve. This is in marked contrast with the lack of correlation obtained in Part I where it was attempted to relate birefringence to elongation parameters. Evidently birefringence is related most directly to internal stress rather than elongation, and in fact provides a direct measure of the internal stress. This simple relation seems to be essentially independent of the stress‐time‐temperature history of the samples during orientation. Measurements on an additional filament which had a very nonuniform birefringence distribution (studied also in Part II) indicate that internal stress should be correlated with area average rather than diameter average birefringence, in cases where such a distinction is necessary. The internal stress‐birefringence curve is not linear, contrary to theoretical predictions based on the simple kinetic theory of rubber elasticity. Non‐Gaussian chain statistics may explain this nonlinearity. The present results indicate that birefringence provides a more useful index of molecular orientation than measurements of total amount of retraction.