Trichoderma viride was batch-cultivated in a stirred tank bioreactor and in a concentric tube airlift bioreactor in order to determine its capacity to remove hexavalent chromium [Cr(VI)] from aqueous solutions. The agitation rate (120 rpm) used during the stirred tank bioreactor cultivation caused mycelial fragmentation, and had a negative effect on the cell growth and Cr(VI) removal. The physical damage inflicted by the agitation rate was, presumably, sufficient to cause fragmentation of mycelia and as a result, diminish Cr(VI) removal. Compared with stirred tanks, pneumatic bioreactors are considered more suitable systems for the cultivation of shear sensitive cells, and hence further studies were carried out in a concentric tube airlift bioreactor. In this type of reactor, the strain exhibited an extraordinary capability to remove high concentrations of the highly toxic and soluble hexavalent chromium. Therefore, the airlift bioreactor might represent an attractive technological alternative for the treatment of wastewaters containing high Cr(VI) concentrations, using the T viride strain. To our knowledge, this is the first report on Cr(VI) removal by a microbial culture in a pneumatically agitated bioreactor. (c) 2006 Elsevier Inc. All rights reserved.