The effect of a shear flow on a colloidal crystalline phase of charge stabilized silica spheres was examined both by theological measurements and by observing the process with a polarizing microscope. The microscopical observations show that a flow breaks up the crystal phase into crystalline regions coexisting with "shear melted" regions. Increasing the flow rate causes the shear melted regions to grow at the expense of the crystalline regions. In theological experiments this transition occurs when on increasing the shear rate a critical shear stress is reached. During the transition the stress does not increase with shear rate until the entire crystalline phase has melted. The critical stress divided by the high-frequency limit of the storage modulus assumes a constant value independent of volume fraction and ionic strength.