We determined the anisotropic optical constants of thin films of a conjugated polymer, the methyl-substituted ladder-type poly(paraphenylene) (m-LPPP), over a large spectral range between 1.55 and 3.5 eV, In the subgap region the anisotropic refractive indices were obtained via waveguiding experiments at 1.55 eV and 1.95 eV. Propagation loss measurements revealed that the absorption coefficient parallel to the polymer film surface is very small (< 12.6 cm(-1)) in this energy region, due to the high purity of the polymer. In the strongly absorbing energy region of the pi-pi electronic transition (2.6-3.5 eV) we performed spectroscopic ellipsometric measurements, in order to determine the anisotropic optical constants parallel (N-o) and perpendicular (N-c) to the polymer surface, The optical anisotropy in the m-LPPP films can be attributed to a preferential orientation of the polymer chains in the plane of the substrate.