We have characterized superstructures of thiacyanine J aggregates in aqueous electrolyte solution in the dye concentration range between 2.5 and 80 mM. Fluorescence microscopy, polarized-light microscopy, and atomic force microscopy proved that the individual thiacyanine J aggregate possessed a mesoscopic rectangular sheetlike morphology (similar to3-7 mum wide and similar to10-30 mum long) consisting of a two-dimensional monomolecular layer of molecules. Synchrotron small-angle X-ray scattering revealed the superstructure of oriented monomolecular sheets which shows a disordered (nematic-like) or an ordered (smectic) layer alignment in solution. These superstructural behaviors can be interpreted in terms of the packing or alignments of the mesoscopic monomolecular sheets of the J aggregate as a function of the dye concentration.