The contribution of one-dimensional exciton states to third-order nonlinear optical (NLO) responses has been investigated on conjugated polymer polydiacetylenes (PDAs). The phase and modulus of complex χ (3)(−3 ω; ω, ω, ω) values have been evaluated for vacuum-deposited blue-form PDA films by means of third-harmonic generation measurements over a wide range of fundamental photon energy. The two-photon resonance effect for the 1Ag g exciton state as well as the three-photon resonance effect for the 1B u exciton state have been experimentally probed. The results are discussed in terms of a three-level model deduced from electro-absorption spectra.