Intermig impellers have been postulated as very efficient for mixing highly viscous non-Newtonian fluids (such as xanthan and mycelial broths). However, no formal characterisation has been published and no fair comparisons have been made, based on accurate power drawn measurements and using equal number of impeller stages and equal diameter, if compared (for example) with the performance of Rushton turbines. Characterisation of the shape, size, and evolution of the well-mixed zones or ＇＇caverns＇＇ were correlated with power drawn, for single and dual Rushton turbines and for one- as well as two-stage Intermig unslotted impellers. Cavern evolution studies were carried out in a mixing tank (diameter = 0.205 m, H/T = 1.6) equipped with an accurate air bearing dynamometer. Carbopol 940 (0.25 wt.-%) was used as a model, transparent fluid. Impeller to tank diameter ratio was 0.53 for both impellers. Caverns were visualised by injecting methylene blue in the well-mixed zones. A single Rushton turbine developed larger caverns if compared with one-stage Intermig of the same diameter under power drawn below 1.5 kW m(-3). At higher power drawn, both impellers behaved very similarly, reaching a limit in cavern volume of about 40% of the total liquid volume, even at very high (20 kW m(-3)) power drawn. A similar trend characterised dual combinations: below 3 kW m(-3), dual Rushtons gave larger cavern volume if compared with the performance of two-stage Intermigs. In either case, power drawn higher than 3 kW m(-3) was sufficient to mix more than 90 % of the liquid volume. The presence or absence of the slot in the Intermig did not influence cavern development. Experiments with a smaller Intermig (D/T = 0.47) showed that, under equal power drawn, caverns were smaller if compared with those obtained with the larger Intermig (D/T = 0.53).