Influence of ionic strength on the steady flow and linear and nonlinear viscoelastic properties of concentrated monodisperse polystyrene latices has been measured by means of a cone–plate type rheometer. In the complete deionized state and in the presence of a small amount of the added salt NaCl, the system, which is notably iridescent, shows a marked non‐Newtonian flow having an evident yield stress and a dynamic modulus which is mostly independent of the angular frequency. This means that a crystal‐like structure of the dispersing particle develops fully in the system. As the salt concentration increases, the iridescence disappears and the system shows Newtonian flow. It is plausible that the crystal–liquid transition begins to occur in the ionic concentration range where the Yukawa potential between the particles is smaller than the thermal energykTandDeff (=2a+2Dl) is close to or less than the distance between neighboring particles in the crystal‐like structure. Hereais the radius of the particle andDlis the Debye screening length. The crystal‐like ordered structure is kept in the nonlinear viscoelastic region of 40% dynamic strain, and it also suggests that the flow and the nonlinear deformation in the colloidal crystal are due to a slip in a lattice plane at a constant stress.