Interactions in a cationic surfactant-hyaluronan system in water and in sodium chloride solution were investigated by high-resolution ultrasonic spectroscopy at 25 degrees C. Two alkyltrimethylammonium bromide surfactants of different chain lengths (tetradecyl and hexadecyl) were used; hyaluronan molecular weight ranged from 10 to 1750 kDa. Two main parameters-ultrasonic velocity and attenuation-were measured in the titration regime. Up to six different regions could be identified in the velocity titration profiles in water in a narrow interval of surfactant concentration. These regions differed primarily in the compressibility of structures formed in the system. The number of detected transitions was higher for the tetradecyl surfactant; therefore, the increased length of the hydrophobic chain simplified the details of the structure-forming behavior. The measurement of attenuation was much less sensitive and detected only the formation of microheterogeneous structures or visible phase separates. The richness of the titration profiles was depressed in salt solution, where essentially only two principal regions were observed. On the other hand, the effect of hyaluronan molecular weight on the positions of boundaries between regions was more significant in the presence of salt. Besides electrostatic interactions, hydrophobic interactions are also relevant for determining the behavior of hyaluronan-surfactant systems and the properties of formed complexes (aggregates).