Third-order nonlinear optical properties of lyotropic liquid crystal poly(1,4-phenylene-2,6-benzobisthiazole), PBT, solutions are studied by third harmonic generation measurements. Besides the enhancement observed for this coefficient with respect to the pure PBT, coupling is observed between the mean filed nematic director n and the incident fundamental polarization B
0. Different geometries, with respect to the relative orientation of B
0 and n, provided different harmonic Maker Fringe Patterns. These data are compared using refractive index measurements, independently achieved on these materials, and necessary for Maker Fringe analysis.