The spectrum of the modulus and the imaginary part of the third order nonlinear optical susceptibility chi(3) of polydiacetylene was studied over the range 750-1000 nm by a combination of two-photon absorption and degenerate four wave mixing measurements. A peak in the two-photon absorption spectrum near 880 nm was found to be due to induced absorption, probably caused by direct optical excitation of the long-lived triplet exciton. No clear evidence was observed for resonances due to two-photon accessible A(g) symmetry states in the spectral range studied.