Interactions between hyaluronan and the cationic surfactant Septonex were studied using high resolution ultrasound spectroscopy. Four different interaction regions were identified in aqueous solution in a narrow interval of surfactant concentrations. In contrast, in 0.15 mol/L NaCl solution, essentially only two principal regions were observed. Hyaluronan-Septonex aggregates were generally less compressible than Septonex monomers or micelles and their formation in water was not appreciably affected by hyaluronan molecular weight. In the presence of NaCl, the effect of hyaluronan molecular weight was more profound, which can be ascribed to its conformational sensitivity to ionic strength combined with its role as a surfactant counterion. Hyaluronan-Septonex interactions were closer to interactions with CTAB than with TTAB. Differences in structure or Krafft temperature (solubility) between Septonex and CTAB were thus not significantly reflected in these interactions. The basic characteristics of Septonex surfactant were also determined.