Batch mixing in the laminar flow regime was studied using single- and double-helical-ribbon impellers and a series of yield stress fluids prepared from the polymer Carbopol 940, water, and sodium hydroxide. Power draw measurements for these non-Newtonian fluids were compared to the values obtained for comparably high viscosity Newtonian fluids. Circulation times in the mixer as a function of impeller speed were measured using the thermal response technique. The dimensionless circulation times were found to be independent of pumping direction and fluid rheology for the double-ribbon impeller but were a function of both these factors for the single ribbon. For the highest viscosity fluid, the single-ribbon impeller with downward pumping gave very high values of dimensionless circulation time. For the single-ribbon impeller, the dimensionless circulation time with upward pumping was less than 2/3 of the value previously observed for Newtonian fluids.