The radial distribution of birefringence, Δ(r), has been measured in Filament # 16 at a series of time values up to 600 hrs, during retraction at 85°C. The birefringence distribution was obtained by cutting wedges on the end of the filament samples and analyzing the optical interference pattern down the center line of the wedges, under a polarizing microscope. The rates of birefringence decay at different points along the radius are not the same, or even proportional; the interrelationship is apparently fairly complex. Birefringence values near the center of the filament change from negative to positive sign in the long‐time region, while values near the exterior surface remain negative throughout. Values of the diameter average birefringence calculated from Δ(r) are in close agreement with values obtained from direct experimental measurement. A cross‐sectional‐area average birefringence was also calculated from Δ(r); this quantity decreases monotonically toward zero, with no transition to positive values in the long‐time region, and therefore corresponds more closely than the diameter average birefringence with the observed length changes. The area average is nearly proportional to the diameter average, however, except for values near zero.