In this paper, the experiments were conducted in a concentric cylinder shear device. The effect on oxidation activity of ferrous iron by Acidithiobacillus firrooxidans was tested under different shear conditions achieved through regulating the rotating speed of the inner cylinder. The results suggested that the fluid shear rate had little adverse effect on the oxidation activity of At. ferrooxidans. In this study, silicon dioxide powder was added to the medium to investigate the effect of particles collision on the oxidation activity of bacterial cells, and it was found that the oxidation rate of ferrous iron by At. ferrooxidans was greatly decreased with the increase of silicon dioxide concentration. (C) 2007 Elsevier Ltd. All rights reserved.