The resonant nonlinear optical response and singlet excited-state dynamics in the thin film of polydithieno (3,2-b; 2′, 3′-d) thiophene are studied by time-resolved degenerate four-wave mixing. Our experimental results, using <1 ps excitation at 635 nm, show that bimolecular decay via exciton–exciton annihilation dominates the temporal response at high intensities, thus decreasing the singlet lifetime. The measured rate constant of the singlet annihilation is k2 = 7.5×10−9 cm3/s. Estimates of the singlet-singlet annihilation, exciton diffusion coefficient, and hopping rate are obtained from the measurements. © 1997 American Institute of Physics.