In this work. the aggregation behavior of the cyanine dye, pseudoisocyanine chloride, in aqueous solutions both in the absence and presence of added electrolyte (0.01 M NaCl) was studied by a combination of UV/ vis spectroscopy and static light scattering (SLS). In the case of pure aqueous dyestuff solutions, an apparent aggregation number of N-app similar to 3 was found. A correction of the apparent molecular weights, M-w,M-app, for the amount of the still present residual monomers gave only slightly larger values (N similar to 4). However, it could be shown that, in the presence of 0.01 M NaCl at low dyestuff concentration (0.75 x 10(-3) mol L-1), the absorption spectrum shows the typical features of what is commonly believed to represent a dimer spectrum, whereas the corresponding SLS data unequivocally prove the existence of larger aggregates, consisting of at least 30 molecules. At higher dyestuff concentrations (similar to2 x 10-3 mol L-1) and in the presence of salt, apparent aggregation numbers of similar to 1000 (uncorrected) and similar to 2000 (corrected for cm) were obtained. A particle form factor analysis of the SLS data by means of the Koyama approximation revealed typical wormlike aggregate structures for the higher concentration solutions. Additionally, the linear mass densities could be obtained at large scattering vectors and from those the number of molecules per unit length. They range from 1 to 2 nm(-1) (uncorrected) and 2 to 6 nm(-1) (corrected for cm) and are consistent with a bundle formation of single aggregated strands. This is in agreement with recent cryo-transmission electron microscopy results obtained for the same compound.