The intra- and/or intermolecular aggregation and the structure of the aggregates formed in water by hydrophobically modified dextrans, prepared by covalent attachment of cholic or deoxycholic acid to dextran of M-v = 30 000, were investigated by various fluorescence and light scattering techniques. From the variation of the fluorescence quantum yield and fluorescence emission maximum of a hydrophobic fluorescent probe, N-phenyl-1-naphthylamine, with polymer concentration, we conclude about the value of the critical aggregation concentration, cac, and the existence of intermolecular aggregation below cac. The values of cac are found to depend on the nature of the hydrophobic moiety and the degree of substitution. The hydrodynamic radii of the aggregates are determined using dynamic light scattering, and the apparent weight-average molecular weights, radii of gyration, and second virial coefficients are evaluated by static Light scattering over a very large concentration range. The results show that below a given critical concentration the hydrophobically modified dextrans form big and loose aggregates and small and compact ones at concentrations higher than 0.2 g %. A transition between the two types of aggregates begins at 0.02 g %, the concentration that we keep calling cac despite the fact that it is the concentration at which the formation of compact aggregates begins rather than that for which intermolecular aggregation occurs. The results are compared with those for unmodified dextran for which no aggregation was observed.