Measurements of the first x(1) and the third order optical susceptibilities X(3) in the spectral region below the band edge of polyacetylene are reviewed. The absorption spectrum in trans-(CH)x has an intense peak due to the onset of the strong interband transition near 2 eV. Inside the semiconducting gap the absorption has an exponential band edge and the transmission losses are dominated by scattering effects. The magnitude of the electronic contribution to the X(3) parallel to the polymer chains in trans-(CH)x is in excess of 10-9 esu for wavelengths larger than 1.3 μ.m, and is one of the largest values for the electronic X(3) inside the gap of a semiconductor. The spectrum of X(3) in trans-(CH)x has a sharp two-photon resonance that has been attributed to the presence of the 21Ag excited state near band edge. It is suggested that through appropriate control of morphology, noncrystalline, processable conjugated polymers can be useful in optoelectronic applications.