Rushton turbines with 6, 12 and 18 blades have been studied under unaerated and aerated conditions in water and 0.8% CMC solutions giving fully turbulent and transitional flow regimes, respectively. In water, as reported earlier, enhanced blade numbers give both a higher ratio of gassed to ungassed power, P-g/P, and a greater air handling capacity at an equal specific power input. However, in the transitional regime, it is found that these advantages no longer exist. This is because, at these Reynolds numbers, the size of the gas-filled cavities is greater and independent of air flow rate. Thus, P-g/P falls to values as low as 0.4, regardless of the number of blades. Comparison with radial flow Scaba 6SRGT impellers in which the flat blade of the Rushton is replaced with one parabolic in cross-section shows that changing the shape is always a superior way of improving impeller performance, compared to increasing the number of flat blades, especially if retrofitting is required.