The third-order nonlinear optical (NLO) properties of a novel pi -conjugated copolymer constituted of electron-rich carbazole and 2,5-diethynylthiophene have been studied using degenerate four-wave mixing (DFWM) at 1064 nm. The off-resonance second-order hyperpolarizability gamma per repeated unit is estimated to be 2.2 x 10(-31)esu; the nonresonant third-order susceptibility chi ((3)) approaches (8.6 +/-1.7) x 10(-10) esu. The large ultrafast nonlinearity originates from an instantaneous electronic polarization.