Dispersion of degenerate four-wave mixing (DFWM) efficiency near the exciton resonance of aggregates of 1,1-diethy1-2-2-cyanine (pseudoisocyanine, PIC) chloride is reported. Correlation with the spectrum of imaginary parts of nonlinearity obtained by pump and probe techniques is discussed. Third order nonlinear susceptibility of PIC aggregates has been calculated supposing highest density packing attainable by Langmuir-Blodgett technique. The nonlinear figure-of-merit of aggregates was evaluated, which is intensity dependent due to exciton annihilation. Observed intensity and wavelength dependence of mixing efficiency is qualitatively explained in terms of absorption saturation, exciton annihilation, and subsequent generation of an excess diagonal disorder by vibrational energy accumulated in aggregates.