The effect of molecular orientation on the stress‐optical coefficient of polystyrene at room temperature (24°C) was measured using polystyrene monofilaments with different degrees of orientation. High degrees of orientation were obtained by cold‐stretching. The optical measurements were made using a polarizing microscope fitted with a Se´narmont compensator. The stress‐optical coefficient shows a strong dependence on molecular orientation, using birefringence as the index of molecular orientation. The stress‐optical coefficient appears to decrease linearlyvsbirefringence, from a value of about +10 brewsters at zero birefringence to a value of about +4 brewsters at −0.04 birefringence. The elastic (Young's) modulus was also measured (in tension) as a function of orientation, and was found to increase nonlinearly with orientation (from 4.3×105psi at zero birefringence to 6.1×105psi at −0.04). A curve of strain‐optical coefficientvsorientation was obtained by multiplying stress‐optical and modulus values; this decreases nonlinearly and to a somewhat lesser extent than the stress‐optical curve (from about +0.03 at zero birefringence to +0.017 at −0.04). The significance of these constants in photoelastic experiments is discussed.