The viscoelastic and thixotropic properties of 30% polystyrene latex with Na–caseinate suspensions were investigated. Oscillatory measurements showed that dynamic rigidities of the suspensions were slightly affected by the addition of Na–caseinate within the range of 1.4% to 5.37%CN(ratio of the weight of added Na–caseinate to the weight of polystyrene latex). The addition of Na–caseinate changed the rheology of the latex suspension from nonthixotropic to thixotropic. The greatest degree of thixotropy was found at theCNof 2.65% which was between the maximum degree of flocculation and the saturation plateau of Na–caseinate adsorption of the latex (CNabout 5%). Empirical equations for constant structure flow curves of the different suspensions were also obtained.