Cryo-transmission electron microscopy (cryo-TEM) has been applied to characterize J-aggregates formed by the dye 1,1'-diethyl-2,2'-cyanine chloride (pseudoisocyanine chloride) in 200 mM sodium chloride solutions as a function of the dye concentration within the range of (2.5-6.1) x 10(-4) mol/L. Electron micrographs reveal the formation of a network superstructure consisting of isolated fibers and complex fiber bundles. Upon dilution, more of the isolated threadlike J-aggregates become visible, which are characterized by a diameter of 2.3 nm, lengths of several hundreds of nanometers and a high stiffness. Such J-aggregates already appear at room temperature at a concentration as low as 2.5 x 10(-4) mol/L. Ends of the threadlike J-aggregates can be found, but they are extremely rare, indicating a large end-cap energy. Because of their morphological appearance, the J-aggregates resemble ordinary polymers rather than "equilibrium polymers". Concentration-dependent growth of J-aggregates cannot be proved. H-aggregates of mesoscopic size are not found.