The linear and third order nonlinear optical properties of a polythiophene derivative incorporating 5,6-dialkoxy-thieno[3,4-b]pyrazine units were investigated. This low band gap material has a large oscillator strength, weak photoluminescence and strong enhancement of the Raman bands. We surmise that the long conjugation length of the polymer backbone, as well as the effect of the substituents on the π-electrons, give rise to a large value of the third order nonlinear susceptibility, χ(3). Measurements by Z-scan and third harmonic generation (THG) corroborate earlier results from degenerate four wave mixing (DFWM) experiments. The χ(3)values from Z-scan with 20 ps pulses were over 10−7esu near the lowest energy absorption band.