The linear and nonlinear rheological properties of a hydrophobically modified guar (hm-HPG) and a surfactant forming wormlike micelles, the erucyl bis(hydroxyethyl)methylammonium chloride, have been investigated in 0.4 M KCl aqueous solution. The comparison between the zero-shear viscosity behaviors of the nonmodified guar and the hm-HPG shows a decrease of the intermolecular associations upon increasing the temperature. Significant synergistic effects were observed between the modified polymer and the wormlike micelles in a range of concentrations extending from 0.1 to 1 wt % and for temperatures up to 60 degrees C. The optimal synergy for the zero-shear viscosity is obtained for mixtures with an overall concentration approximate to 0.35 wt % and a surfactant/polymer ratio equal to 1/4 (w/w). The terminal time of the stress relaxation, the zero-shear viscosity as well as the viscosity at a shear rate of 100 s(-1) and the lifetime of the associations show a maximum at this composition. The experimental observations suggest the formation of very efficient cross-links between the wormlike micelles and the polymer chains.