The third order nonlinear optical susceptibility has been measured by degenerate four wave mixing in a 33‐&mgr;m‐thick biaxial film of a conjugated aromatic heterocyclic polymer, poly‐p‐phenylenebenzobisthiazole, commonly known as PBT, which has a very high mechanical strength due to its rigid rod conformation as well as environmental stability and a high laser damage threshold. For the first time, the response of the optical nonlinearity in the &pgr;‐electron conjugated system has been experimentally verified to be in subpicoseconds. The value of &khgr;(3)is found to be about an order of magnitude larger than that of CS2. The measurement at two different wavelengths suggests that they are nonresonant &khgr;(3)values. The measured anisotropy of &khgr;(3)as a function of angular orientation at two different sets of laser polarization is explained by using the tensor properties of &khgr;(3)in an anisotropic medium.