Squarylium dye J aggregates exhibit ultrafast nonlinear optical response of absorption saturation at the resonant wavelength of 770 nm. We studied the two-photon excitation process of J aggregates. By fluorescence measurement, we found the two-photon absorption band at 1.3 μm, which was different from that of the dye solution at 1.2 μm. Absorption saturation at 770 nm via a two-photon excitation process was observed by two-photon resonant excitation at 1.3 μm and also by off-resonant excitation at 1.55 μm, suggesting the possibility of J aggregates for optical switching materials working at the wavelength used in optical communications. © 2001 American Institute of Physics.