Fabrication and femtosecond nonlinear optical response of Langmuir-Blodgett (LB) films with two-dimensional J-aggregates are described. Single-layer LB-films were deposited on glass substrates from mono-molecular layers of squarylium dye J-aggregates at an air-water interface (Langmuir film). To obtain EB-films containing J-aggregates, deposition of the Langmuir film with compression slightly in excess of that required for monolayer occupation was necessary. Coating the surface of the LB-Films with a glassy perfluorocarbon polymer soon after deposition was effective in enhancing the formation of J-aggregates and stabilizing their structures. The LB-films of squarylium dye J-aggregates exhibit a single J-band at 783 nm with an extremely large absorbance of 0.2/monolayer, which is comparable to that of the Langmuir films. The morphology of J-aggregates in the LB-films was characterized by transmission images obtained using a near-field scanning optical microscope (NSOM) at a resonant wavelength of 780 nm. The LB-films were comprised of randomly polarized two-dimensional domains in which transition moments of J-aggregates were oriented in a single direction on a sub-mu m scale. Ultrafast nonlinear optical responses were characterized by means of resonant pump-probe measurements. The observed low absorption-saturation energy (3.4 mu J/cm(2). pulse) and ultrafast relaxation (time-constant of 200 fs) are indicative of delocalized excited states within highly organized structures of the squarylium dye J-aggregates in the LB-film which preserves the two-dimensional mono-molecular layer structures initially formed in the Langmuir films. Stable, solid-substrate-fixed, two-dimensional monolayers thus fabricated are promising for the realization of ultrafast planar all-optical switching devices.