THE photo-elastic properties of polystyrene at temperatures above its softening point have been investigated earlier1,2, and it has been found that under these conditions the stress-birefringence is in the opposite direction from that found for most other materials. This is in agreement with the results reported by Signer3 on the flow-birefringence of polystyrene solutions. Signer pointed out that this behaviour would be expected if it is assumed that, when the molecular chains become aligned, the benzene rings orientate themselves at right angles to the direction of the chains; and, since the optical effect of the benzene rings is greater than that of the main chain, the net stress-birefringence will be negative.