Unlike conventional stirred tanks, rotor-stator mixers provide high deformation rates to a relatively limited volume, resulting in a region in which intensive mixing, milling, and/or dispersion operations can occur. FLUENT was used to conduct three-dimensional CFD simulations of the IKA prototype mixer, an in-line slot and tooth rotor-stator device, for a low-speed low-flow condition and a high-speed high-flow condition. The main objective of this project was to develop a CFD model of the IKA prototype mixer with the necessary refinement in the shear gap to accurately resolve these high shear values. A grid independence study was conducted to quantify the influence of shear gap grid resolution on the computed flow solution and determine the grid level most suitable for further detailed investigation. Convergence in highly-directed regions was shown to be faster than in more-open regions. Velocity and total deformation fields in the stator slots and the shear gap were examined for both operating scenarios. Differences in the fluid behaviour between the two scenarios are discussed.