We report on the demonstration of the femtosecond nonlinear optical response from a two-dimensional monomolecular layer of squarylium dyeJaggregate at 5 °C. The formation of a monomolecular layer Langmuir film was achieved by spreading squarylium dye modified by two propyl and two hexyl groups at the air–water interface, which resulted in a very strongJband(o.d.=0.3)at 777 nm. The transient absorption spectra in a resonant pump-probe measurement showed a low absorption saturation power(9.7×106 W/cm2)and an ultrafast response (300 fs), which are indicative of exciton delocalization over 18 molecules in thisJaggregate, even at 5 °C. ©1998 American Institute of Physics.