Third harmonic generation (THG) is used to study the third-order nonlinear optical properties of nematic and isotropic solutions of poly(1,4-phenylene-2,6-benzobisthiazole) and related small-molecule model compounds. Maker fringe patterns (MFP) obtained using both plane slab and wedge-shaped cells for fundamental wavelengths of 1.542 and 1.907 mu m are analyzed in terms of postulated surface layers in the isotropic preparations. The layers are postulated to comprise regions with concentration smaller than for the bulk sample, in which the rod-like chains tend to have their axes parallel to the plane of the surface, creating a negatively birefringent uniaxial nematic layer. The THG with the nematic solution exhibits intensity with polarization components unexpected for the uniaxial nematic symmetry attributed from linear optical properties, along with other unexpected features in the MFP. This behavior is attributed to the effects of surface layers, postulated to exhibit biaxial nematic symmetry, in which the uniaxial nematic symmetry is broken in regions on the scale of a wavelength. This provides for averaging that does not cause departure from the linear optical behavior expected for a uniaxial nematic, but does produce effects on nonlinear optical properties.