Cryo-transmission electron microscopy (cryo-TEM), absorption spectroscopy with polarized light, and polarized-light optical microscopy have been used to characterize the J-aggregates formed by the dye 1,1'-diethyl-2,2'-cyanine chloride (pseudoisocyanine chloride) in aqueous solution. Cryo-TEM visualizes for the first time directly the rodlike morphology of the J-aggregates. A rod diameter of 2.3 +/- 0.2 nm is estimated. Absorption spectroscopy shows that J-aggregation is a strong function of dye concentration and starts in dilute solution before the viscosity increases and mesophases appear. For a 12.5 x 10(-3) mol solution, the length of the J-aggregates is on the order of 350 nm, which corresponds to aggregation numbers of approximate to 3000. Optical microscope textures reveal columnar nematic and hexagonal phases at higher dye concentrations. Structural alternatives for the molecular packing within the J-aggregate based on the estimated rod geometry are discussed. A quasi-two-dimensional superstructure is proposed which could better explain the optical properties of the J-aggregates than previous models.