The interaction between sodium hyaluronate (NaHy) and decyltrimethylammonium bromide (C(10)TAB) has been investigated by phase behavior, viscosity, and dynamic shear moduli studies. When the concentration of C(10)TAB, C-s, was between 0.03 and 0.35 M and the polymer concentration was above 0.05 g L-1, the system separated into two phases. For the isotropic solutions containing a large excess of surfactant (C-s greater than or equal to 0.35 M), the boundary between the dilute and semidilute regime was determined by using the zero-shear specific viscosity data. Further, the intrinsic viscosity and the dynamic shear moduli of NaHy in C(10)TAB solutions were compared with those in NaCl solutions. The viscosity results suggested that NaHy and C(10)TAB formed compact complexes in 0.35 M C(10)TAB to give a small intrinsic viscosity and the size of the complexes increased with the increase of C(10)TAB concentration. When the added salt concentration was 1.00 M and the polymer concentration was in semidilute regime, it was suggested from the data on dynamic shear moduli that the entanglement density of Hy chains in C(10)TAB was higher than that in NaCl.