Polyacrylamides hydrophobically modified with a small amount of dihexylacrylamide (HMPAM) have been prepared by means of an aqueous micellar polymerization technique. This process leads to copolymers in which the hydrophobic molecules are randomly distributed as blocks in the acrylamide backbone. Keeping constant the hydrophobe level (1 mol%), the length of the blocks was adjusted by varying the surfactant content used in the synthesis, i.e. by varying the number of hydrophobes per micelle, N-H The resulting copolymers are homogeneous in composition, but have a different microstructure (blockiness). The rheological properties of the samples in aqueous solution have been investigated as a function of polymer concentration, polymer conversion, temperature and in the presence of surfactant, using steady flow and oscillatory experiments. The hydrophobic modification of the polyacrylamide backbone induces a slowing down of the reptation movement of the polymer chains. The results stress the major role of the hydrophobe distribution in the copolymer for a good control of the thickening efficiency of associating HMPAM.