The aggregation of chitosan (CS) has been studied as a function of concentration, degree of acetylation (DA), and degree of polymerization (DP) by means of pyrene fluorescence and rheology. Fluorescence experiments show that aggregation of CS involves hydrophobic domains (HD) which are more favoured as lower the DA and DP. Consistent with these results, the viscosity of CS solutions decreases continuously on increasing DA, in the whole range of DP. These results, which rule out the participation of the acetyl groups in the HD, have been interpreted by the theory of hydrophobic polyelectrolytes in terms of the electrostatic energy of the aggregates.