The rheology of aqueous solutions of a hydrophobically modified cationic hydroxyethylcellulose ether (LM200) in the presence of hydrophobically modified poly(sodium 2-acrylamido-2-methylpropanesulfonates) (HM-PAMPS) was studied in the semidilute regime (10 g L-1) by oscillatory shear and steady shear measurements. The polyanions were copolymers of sodium 2-acrylamido-2-methylpropanesulfonate and N-n-dodecylmethacrylamide (PA90Dod or PA80Dod) with dodecyl group content of 10 and 20 mol %, respectively. The LM200/polyanion mixtures underwent phase separation for compositions near charge neutralization but were homogeneous over wide composition ranges. Miscible fluids with excess polyanion with respect to the polycation content behaved as elastic gels, viscoelastic fluids, or viscous liquids, depending on the mixture composition and the level of hydrophobic modification of the polyanion. The viscous polyanion-rich fluids were Newtonian fluids at low shear rates; they increased in viscosity at moderate shear rates (shear thickening regime) and underwent shear thinning at high shear rates. In contrast, polyanion-poor mixtures were viscoelastic fluids and exhibited shear-thinning properties for all compositions and independently of the dodecyl content of the polyanion. The occurrence of two types of networks in LM200/HM-PAMPS mixed fluids is attributed to the disparity in the charge density, level of hydrophobic modification, and flexibility of the polycation and polyanions.