The effect of solids (up to 30% w/w) on the viability of a mesophilic culture of acidophilic bacteria was investigated in stirred tank reactors (STRs) using the Rushton turbine (RT) and the pitched blade turbine impellers in a speed range of 2.01-3.35 m/s. The results showed that hydrodynamic shear alone as a characteristic function of impeller type and speed has a very limited effect on the bacterial cells during mixing in STRs, but mechanical damage to bacterial cells occurs, to a most significant extent, via the attrition by solid particles promoted by the intensity of agitation. Extent of the adverse effect on the bacterial cells was found to depend on impeller design/speed and solids density. The loss in the viability of bacterial population with a tendency to increase with agitation rate and solids density was more extensive with the RT than the PBT impellers under the same experimental conditions e.g. 72% loss in the viability c.f. 40% over 4 h of mixing at 20%w/w solids and 3.35 m/s impeller speed. The kinetic analysis of the experimental data suggest that the rate and extent of oxidation of a substrate in a given process would be controlled by the inoculum size and by the difference between the "normal" growth rate and the deactivation rate of bacterial cells incurred at a particular mixing condition (impeller type/speed and solids density) i.e. by the "actual" growth rate. (C) 2002 Elsevier Science Ltd. All rights reserved.