The mixing of yield stress fluids with an anchor impeller is investigated numerically in the laminar regime using a three-dimensional finite element method, First, it is shown that the flow behavior is strongly influenced by the level of viscoplasticity and that mixing is rather inefficient in such a case, Next, correlations of the power number versus the Reynolds number are presented for different values of the Bingham number, In particular, it is established that the Metzner-Otto concept remains valid for fluid exhibiting a yield stress and that the Metzner and Otto constant (K-s) varies slightly over a wide range of Bingham numbers (Bi less than or equal to 7500). Finally, the 25-year old experimental findings of Nagata are reviewed and clarified.