We have examined the structure of hydrophobically modified chitosans in a selective aqueous solution at T = 25 degreesC, i.e., a nonsolvent for the alkyl grafted chains and a good solvent for the polyelectrolyte chitosan backbone. The behavior of the hydrophobically modified chitosan was investigated using viscosity and static and dynamic light scattering. We observed three regions on the phase diagram of the associative polymer in aqueous solution: (i) a supernatant phase (unimers phase) at low polymer concentration; (ii) solutions of intermolecularly bridged "flowerlike" micelles at intermediate polymer concentration; (iii) an associative gel phase at high polymer content. The aggregation process in dilute regime was investigated using combined static and dynamic light scattering. In particular, the concentration dependence of the aggregation number, of the micellar aggregates size, of the micelle concentration, and of the unimer concentration is discussed.