The nonlinear optical properties of pseudoisocyanine J-aggregates in Langmuir-Blodgett films have been investigated by means of degenerate four wave mixing (DFWM) process. The conversion efficiency of the DFWM signal and the estimated third order susceptibility were found to be comparable with those of pseudoisocyanine J-aggregates in solution under comparable resonance conditions. The intensity dependence of the converted (DFWM) signal is interpreted as resulting from different processes including saturation, exciton-exciton annihilation and secondary processes of dynamical disorder.